M81 and M82 Image © Gimmi Ratto. Used with permission.
This FAQ for the Digital Astrophotography is based on one originally generated by Simon Szykman around 2003. It has been significantly updated to reflect changes in this area since 2003. This current version is dated December 2005. While I have based this update (as was the original) on much of the material that has been discussed over the years in the Digital Astrophotography group on Yahoo, all errors and omissions are mine. If you find things that need adding or changed, please email Teri Smoot (tas_astro@yahoo.com).
M81 and M82 Image © Gimmi Ratto. Used with permission.
Almost any of the newsgroups dealing with astronomy will not have discussions regarding astrophotography. Threads related to use of digital cameras are becoming more common on the sci.astro.amateur newsgroup. However, perhaps the most focused forum for discussions on astrphotography is the digital_astro Yahoo! Group. As of the end of 2005, this community has over 10000 members and averages almost 2000 posts each month on topics related to Digital Astrophotography.
The kinds of cameras that can be used for digital astrophotography include simple point and shoot cameras, the more professional Digital Single Lens Reflex type of cameras, webcams, and CCD cameras. There are also other types of dedicated cameras such as the Meade DSI. This FAQ will focus on the point and shoot cameras and on DSLR's. And, as DSLR prices have come down over the last several years, these are the digital cameras that the better digital camera astrophotographers are settling on.
In the past several months, images taken with digital camera images have appeared more frequently than those from film cameras in the Sky and Telescope magazine Gallery pages (CCD images have appeared most often though). Furthermore, Kodak has announced that they are discontinuing their Technical Pan film (red-sensitive, fine-grain, and high-contrast). All of this is a clear indication that, in some cases at least, images from digital cameras can now compete equally with film images and are actively challenging those taken with CCDs. The digital_astro Yahoo! Group has monthly challenge imaging competitions. The entries in these challenges are certainly among the best images group members have produced. A number of images from monthly challenge winners appear throughout this FAQ. A complete gallery of all of the images entered into the challenges for the last six months or so can be found at http://digitalastro.skyinsight.net/gallery/. This should provide some idea of how good digital camera images can get.
There is no cut-and-dry answer to this question. What you can image, how much you can magnify it, and how much detail you can capture depend on the capabilities of the digital camera you have; prices for digital cameras range from the order of tens of dollars to the order of thousands of dollars. Camera capabilities vary with price. Low end cameras have exposure times limited to under a second, and allow little to no control over exposure settings; mid-range cameras provide significant control over exposure settings, and allow exposure times of several to many seconds; high-range cameras have even higher-quality CCD or CMOS chips and allow exposure times of multiple minutes.
Just as important is the astronomical equipment you have. Any digital camera, no matter the quality, will be limited to the light that reaches it. As with visual astronomy, the light-gathering ability of your scope, quality of scope and eyepiece optics, and seeing conditions all affect what comes out of the eyepiece.
Even inexpensive digital cameras can be quite capable of taking snapshots of the moon. With most cameras you will be able to capture an images of the brighter planets, but what you can see in those images depends on your equipment. People have also been able to image a variety of deep-space objects, but this requires a scope with an aperture large enough to see them in the first place, and typically needs a camera that provides longer exposure times and some control exposure settings. The use of image stacking helps (and in some cases is essential) for most Deep Space Objects (DSO's), and performing digital image processing techniques after imaging contributes considerably to the quality of the results.
And, when you start exposing for longer intervals and stacking multiple images, the quality of the telescope's mount becomes much more important.
Your mileage will vary depending on a variety of factors. These will be mentioned only briefly here, as they are discussed in greater detail in other documents that this FAQ links to. At a high level, aside from the standard variables that apply to anyone with a telescope (seeing, darkness of viewing site, quality of optics, etc.), the main factors that affect the results of your venture into digital camera astronomical imaging are:
1. Size of telescope aperture
More light coming in is always better. While this is less of an issue for lunar imaging and only slightly more important for planetary imaging (since a large aperture will capture an equivalent amount of light in a shorter exposure than a small one), it is critical for any attempts at deep sky object imaging. Note that your field of view will likely be smaller if the aperture is larger. If you are trying to image large objects, you will also need a short focal length. Also, you can compensate for aperture somewhat by taking longer exposures if your mount supports that.
2. Whether or not you have a tracking telescope
Without a tracking telescope, longer exposures will start to blur because of the apparent motion of the sky resulting from the Earth's rotation. The degree of blur increases with level of magnification, so that at high magnifications you may be limited to exposures of only a few seconds even if your camera is capable of longer ones. In addition, the quality of your telescope's mount, even if it is of a tracking variety. needs to be sufficient to not smear the images during the exposures.
3. Maximum exposure time
Virtually any camera will be able to take pictures of the moon. To take pictures of bright planets, exposure times of fractions of a second to several seconds will be useful. For DSOs, maximum exposure times of 8 seconds will put only a few within reach, but 16 seconds or longer is really needed to expand your options beyond a small handful. To start collecting quality data on galaxies and the dimmer nebula, these exposure times need to grow to minutes.
4. Other features of your digital camera
Maximum resolution will be limited to the number of pixels your digital camera has, but resolution is often limited by your astronomical equipment and seeing conditions rather than by the resolution of your camera. More important is the feature set your camera provides, as that will lead to more flexibility in what you can achieve.
5. Ambient temperature
Since digital camera CCDs are not cooled, ambient temperature can cause noise in the images. (The issue of noise is discussed in greater detail in a separate question.)
6. Camera mounting
Whether your camera is hand-held, tripod mounted, or coupled to your telescope has a significant effect on what you do with it. Some mounting methods have advantages over others (including stability and alignment), so even for a mounted camera, the method of mounting can make a difference to some extent.
7. Image processing
The use of digital image processing techniques can significantly improve the images you produce. A good shot of the moon may not need much in the way of image processing--perhaps only a bit of contrast enhancement and a little sharpening. At the other end of the spectrum, for DSOs, stacking and other forms of image enhancement can do wonders.
8. Technique
This FAQ is intended only to provide a starting point, and in no way provides a recipe for perfect results every time. A willingness to learn from others, willingness to experiment and patience on your part will go far.
M42 Image © Gerald Wechselberger . Used with permission.
If you go to your local electronics store and ask to see a digital camera, you will almost certainly be shown a "Point and Shoot" (P&S) digital camera. These digital cameras have a fixed (i.e., non-removable) lens and lots of automated features. They are designed for general use photography. They can be used for astrophotography and very good astrophotography can be done with these if they have enough manual features (see the discussion below). However, you will be limited to using the lens that came with the camera.
While you can find a SLR (single lens reflex) camera in your electronics store, these will be more likely sold and supported by a camera/photography store. These cameras tend to be used more by professionals and feature detachable and interchangeable lenses and much more manual control. If you watch sporting events or news shows, the cameras that the professionals use will tend to be SLR's. A digital SLR (or DSLR) is a digital rendition of this type of camera and uses either a CCD or CMOS sensor for imaging rather than film.
While both P&S and DSLR cameras can both do astrophotography, the quality of the image (especially for DSO's) will be better and good images will be easier to obtain with a DSLR. DSLR's in common usage at this time are the Canon 10D, 20D, the Canon Digital Rebels (300D and 350D), as well as the Nikon D70. DSLR's will also tend to cost more than a P&S camera (although there is quite a bit of price overlap).
Afocal photography consists of taking an unmodified Point and Shoot camera focused at infinity and taking a picture through the telescope's eyepiece. Mounting the camera to the scope itself is helpful with this method, but for short exposure times images can be obtained simply by holding the camera up to the eyepiece.
With prime focus photography, the camera lens is removed from the camera (which means that the camera is usually a DSLR), the eyepiece is removed from the telescope, and the scope itself serves as the lens for the camera. Because alignment is more important, to use this method the camera must be mounted to the telescope (which generally is a tracking telescope).
Eyepiece projection photography again uses a camera with the lens removed, but this time uses an eyepiece in the optical path. This results in higher focal length, and greater magnification. Magnifying the image results in less light falling on a given area, which translates to a need for longer exposures than prime focus photography given the same target (or for imaging brighter targets). Also because of the higher magnification tracking precision is more important than with prime focus photography.
In the realm of digital cameras, only the high-end professional grade cameras and DSLR's have removable lenses. As a result, afocal photography is essentially the only method available for Point and Shoot camera owners. Although the best results you can get with afocal photography may not be as good as with prime focus or eyepiece projection photography, afocal photography is the easiest method to get started with.
Light coming out of the eyepiece creates a cone. The light then should then strike a properly-aligned surface. In the context of astronomical imaging, vignetting occurs when this illuminated area does not fully cover the light detecting surface (the CCD chip in a digital camera). Because the image does not completely cover the CCD chip, the result is a circular image that is smaller than the camera's field of view. This effect, which causes a dark area surrounding a smaller circular image, is called vignetting.
The cone of light narrows as distance from the surface of the eyepiece increases. Thus, as a camera lens is moved closer to the eyepiece, the cone of light intersected has a larger diameter, resulting in a larger illuminated area reaching the CCD chip. In most cases, to avoid vignetting, a P&S camera should be placed as close to the eyepiece as possible. However, when using eyepieces with relatively long eye relief it is actually possible to reduce the field of view by having the camera too close to the eyepiece. In some cases, the best placement for the camera may be a bit farther away from the eyepiece rather than as close as possible. If you have eyepieces with long eye relief, you may have to experiment a bit to find the best distance. Once the camera has been mounted, use of full zoom will often also be helpful in reducing the effects of vignetting.
Vignetting is not as much of a concern when using a DSLR at prime focus. However, the optical properties of the telescope and camera can cause an uneven illumination over the sensor in the DSLR. This uneven illumination is usually compensated by taking and applying a flat frame (see below).
There's more technical terminology and jargon that can be included in this FAQ. For astrophotography-related terminology, a good concise glossary of terms can be found at Starizona's web site. If you don't find what you are looking for there, a more comprehensive glossary is available at NASA Goddard Space Flight Center's web site. If you are looking for digital camera-related terminology, good glossaries can be found at Radio Shack's web site or ACD Systems' web site.
Alternately, ask a question at the Digital Astro site. Someone there will most likely be struggling with the same issue and you will likely find another person who is happy to help you and explain a specific term.
Mars Image © Daniel Ethier. Used with Permission.
Searching individual digital camera manufacturer sites for information can take a while if you don't know what you are looking for. It is definitely easier to search sites geared toward people interested in buying digital cameras. These sites have more information in one place, can help you find products based on your needs, allow product comparisons, provide equipment reviews, and more. There are many such sites on the web. The ones I found myself using the most are imaging-resource.com, dpreview.com,and steves-digicams.com. All three have general camera information, in-depth reviews, and discussion forums. The first two also let you do side-by-side camera comparisons, with imaging-resource.com providing the more comprehensive comparisons in terms of camera features. The dcviews.com web site has also been recommended.
There are so many cameras out there that you need to decide on what basis you are going to narrow things down. Unless money is no object, the best way to start is to decide on your budget ahead of time. Keep in mind that in addition to the camera, you will also need to spend money on other camera accessories, as well something with which to mount the camera to your scope. That should narrow things down to a more manageable number of choices. Reviews, side-by-side feature comparisons, and budget constraints should help you prune your list from there.
The Digital Astro list is also a very good place to go if you plan on using the camera mostly for astrophotography. If you look at the archives, you can see what others have said about that camera and astrophotography. You can also look at the challenge entries to get some ideas as to how each of the cameras work.
The camera spec that is almost always the first one mentioned is the number of megapixels. This refers to the number of millions of pixels on the camera's imaging chip. This certainly is important, but there are quite a few reasons why there is more to a camera than the number of megapixels it has. For a given price, there will always be tradeoffs. A full-featured 2.3 megapixel camera may cost the same as a bare-bones 3.1 megapixel camera, but the extra pixels in the latter may not make up for the limitations that the lack of features causes.
Secondly, your ability to resolve detail depends on the equipment you are using (mainly on aperture) and seeing conditions. Thus, it may be these factors, rather than the number of megapixels on your camera's CCD, that limit the resolution in your images. Along similar lines, even modest digital cameras generally have better resolution than computer monitors, so if you only plan on viewing images on your computer or putting them on the web, the images as displayed will not show the full resolution of the image itself. If you plan on printing out images and enlarging them, that's a different situation. But the basic point here is that there's no use in paying more for a "better" camera if the resolution provided by your telescopic or computer setup won't benefit from additional megapixels.
In addition, with megapixels as with anything, it's not just quantity that matters, but also quality. Manufacturers can increase number of pixels, but if it's done without increasing the size of the CCD chip, the pixels will by necessity be smaller. The main benefit of a smaller pixel is that it improves resolution; you will, in theory, be able to resolve smaller features in the subject of your image. However, there are also drawbacks to smaller pixels.
Depending on the quality of the CCD chip, smaller pixels can be less sensitive than larger ones. They can also be more prone to noise. This may sound odd given the previous statement, but it isn't necessarily a contradiction. Remember that you want the pixel to brighten due to incoming light, and noise is caused by a pixel that brightens due to energy that has some source other than incoming light (thermal, electrical, cosmic rays, etc.). A pixel may have lower sensitivity to incoming light (which is what you want to detect), but may be more prone to noise because a fixed amount of unwanted energy can cause more brightening in a smaller pixel than in a larger one.
Aside from noise issues, smaller pixels can also be more sensitive to blooming. When one pixel becomes fully saturated (sometimes due to noise, but more often due to incoming light from a particularly bright portion of the target), electrical charge can actually leak into neighboring pixels. Because the additional charge on neighboring pixels is due to leakage rather incoming light, those neighboring pixels brighten more than the would from light alone. This phenomenon is referred to as blooming.
Lastly, being over-equipped can even be counterproductive when you consider the fact that more pixels also translate to larger image file sizes (other factors such as level of image compression being equal), which means you'll be able to store fewer images on a given memory card. It also means that you will need more memory and hard disk space for processing of the images.
All this is not to say that more pixels are bad, or that smaller pixels are bad. Fewer pixels and larger pixels both reduce resolution in images, so more and smaller can be good. The thing to keep in mind is simply that more is not necessarily better, and that other issues such as CCD quality, sensitivity, and general limitations due to viewing conditions, will all affect what will give you the best results with your equipment. MicroPublishing News published a nice article on the megapixel issue.
The first thing you need to do is consider your budget. There are plenty of useful features, but depending on your price range, they may not all be within your reach. You should also consider your intended usage. What else are you going to use your camera for besides astronomical imaging? If the answer is "nothing" then you may want to consider buying a more specialized astronomical CCD camera instead, but most digital camera astrophotographers want (or already have) a digital camera for regular photography as well. I will be focusing on features that are relevant to astronomical imaging, but there may be other features you are interested in for separate reasons.
1. Manual Focus
One of the more important features to look for in a P&S camera is manual focus. Fortunately, this feature is available for all but the cheapest of cameras, even those which do not provide fully-manual control over other exposure settings. Cameras that do not have manual focus setting but that do achieve focus through the lens (TTL focus) may still be able to focus properly on large clear targets (the moon, and possibly magnified planetary views) using the built-in autofocus. For dimmer or less well-defined targets, a camera may have trouble identifying the target and focusing on it. Cameras that do not have manual focus and accomplish focusing with a sensor that is mounted on the camera body (not going through the camera lens) will very likely have serious focusing problems. Their line of sight will be aimed at the outside of the scope rather than up at the sky, and will have a close focus rather than focusing at infinity.
For DSLR's used at prime focus, you really don't need to worry about the camera's focus since you will not be using a lens and you will be using the telescope's focuser to focus on objects. If you want to do piggyback imaging with a DSLR, though, you will need a lens that supports manual focus.
2. Control of Exposure Settings
The next thing to look for is some degree of manual control over exposure settings. Manual control can include one or more of the following: shutter priority (where you manually set the exposure time and the camera sets the aperture automatically), aperture priority (where you manually set the aperture and the camera sets the exposure time automatically), or fully-manual capabilities where you can control both. Of the two priority settings, for astronomical imaging shutter priority is more useful than aperture priority because control over the duration of an exposure allows you to have some degree of control over noise (which increases with longer exposures) and blurring (which can occur as a result of non-ideal seeing conditions as well as due to the earth's rotation depending on your scope's tracking capability). A camera that does not have aperture priority or a fully manual mode will always automatically set its own exposure lengths based on its own internal algorithms. If you have a camera that that does light metering through the lens (TTL light metering), it may still be possible to get good images, but as the aperture-setting algorithms vary from camera to camera, it would be hard to predict results for a given camera without trying it out. If you are considering a camera that does not give any user control over aperture, it would be best to try finding other people who have tried using that particular model camera for astronomical imaging, and ask about their results, before purchasing it. A camera that does not do TTL light metering but instead uses a photocell mounted elsewhere on the body of the camera will not be usable for taking images of the moon. Without metering light through the lens, the photocell will detect only the darkness in front of the camera and will always take exposures that are much longer than what you need for the bright moon, resulting in overexposed images.
Again, the issue of aperture priority is not as much an issue with a DSLR. Manual control of the exposure time is, however, critical.
Image © Philipe Lange. Used with permission
3. Exposure Times
As indicated previously, the moon makes an easy target even for cameras having very limited capabilities. If you have an interest in experimenting with targets other than the moon, exposure time is an important consideration. Be aware that different cameras have a broad range in maximum exposure times--some have maximum exposure times that are 1/2 second or less, while others have maximum times in the multiple seconds, and others up to several minutes. Virtually any camera will be able to take pictures of the moon. To take pictures of bright planets, exposure times of fractions of a second to several seconds will be useful. For DSOs, maximum exposure times of 8 seconds will put a few within reach, but 16 seconds or longer is really needed to expand your options beyond a small handful and exposure times of minutes are required to get most of the dimmer ones. Longer exposure times tend to result in more noise in the images. However, newer cameras have higher quality CCD chips that are much less noisy. In addition, some cameras have built in noise reduction (dark frame subtraction) to reduce noise, and for those that do not, dark frame subtraction can be done as part of digital image processing.
Look for the words "bulb" mode when you are considering a camera. This is an old term but it means that the camera can take extended exposures by holding the aperture open. For modern DSLR's, exposure times can, essentially, be infinite (other issues will limit the exposure time).
4. Lens
Another thing to consider is the size of the lens on the digital camera. Recall that the light from an eyepiece illuminates a circular area on the lens, which then gets focused onto the CCD chip in the camera. Once the diameter of that circular area becomes smaller than the diameter of your lens, the focused light will no longer fully illuminate the CCD chip, which results in vignetting. Vignetting can become a problem with any camera, but as a general rule, the larger the lens on a digital camera is, the more easily you will run into the problem of not being able to fully illuminate the lens. At exactly what point that happens depends not just on the camera, but on your entire setup (scope/eyepiece/camera). In doing camera comparisons, if you are leaning toward a camera whose lens size is not toward the smaller end of the spectrum, it is probably worth doing a bit of asking around to see if you can find somebody else with a setup similar to yours who has had experience with that particular camera.
If you are doing prime focus with a DSLR, the issue of lens size and vignetting will not be a concern. You can, however, still get variations in illumination across the imaging sensor itself. You will generally use a flat field to eliminate this.
5. Optical Zoom
While on the subject of vignetting, you will also want to consider the amount of built-in optical zoom a camera has, since taking pictures at full zoom is generally useful if not necessary for avoiding vignetting. However, it is important to note here that more is not always better. Cameras with more zoom capability may have more optical elements in the light path, so a high zoom can cut down on the amount of light that reaches the CCD. And unlike an external zoom lens which is removed when not in use, a built-in zoom lens cannot be removed. Light will therefore always have to pass through all the optical elements in the camera whether you are at full zoom or not. Note that this discussion involves optical zoom, rather than digital zoom. Though digital zoom can help a bit with focusing, it is not useful for imaging. Unlike an optical zoom, a digital zoom enlarges an image at the expense of reducing resolution. This is generally undesirable in astronomical images. Furthermore, if you do want to enlarge an image digitally despite the resolution loss, it can always be done later using image processing software.
5. Mounting Method
As mentioned earlier, a camera can be hand-held, tripod mounted, or mounted directly to your scope. If you want to use a tripod mounted camera, your camera should have a tripod mounting hole. If you want to couple your camera directly to your scope, it is advisable to look for a camera that has a threaded lens barrel, which will allow you to couple your camera to your scope with a high degree of alignment. Adapters that are made specifically for cameras that do not have threaded lens barrels can be purchased or built, but several of them do require a tripod mounting hole. These can provide respectable results; however, achieving a very good alignment between the camera's optical axis and that of the scope is much more difficult with these systems. Poor alignment can increase the amount of vignetting in your images, it can affect the way light reaches your camera (leading to reduced brightness or non-uniform brightness) and can affect uniformity of focus which in turn impacts the sharpness of your images.
6. Hands-off Imaging
If you plan on taking images of anything besides a bright moon, you will likely move into exposure times for which camera motion can blur images. Even very small motions can affect the sharpness of an image. Mounting your camera to your scope or to a tripod helps, but even the act of pressing the button to capture an image can cause some motion. As a result, a self-timer is a useful feature to have. With this feature, pressing the button starts a timer that captures the image after a delay, which allows motion and vibrations to die out. Even better than a self timer is a camera that has a remote control as an accessory (either included or sold separately). This provides more control over exactly when you capture an image, as well as more convenience. Also, the question of whether or not vibrations have completely died out is less of an issue since you are pushing a button that is not located on the camera. For a number of cameras on the market, freeware or shareware software is available to remotely control a camera using a computer. As a result, some people have taken to making a laptop part of their imaging equipment.
7. In-camera Image Processing
Lastly, for astronomical imaging it is useful to have control over image compression and in-camera image processing. Many cameras have several image quality settings that the user can choose from (higher quality is the result of doing less image compression, at the cost of larger image file sizes). Some cameras even allow images to be saved in TIFF or RAW format, which uses no compression (though it results in correspondingly large files). Final images can always be compressed later on, but if you plan on doing any image processing, the less compression done on the pre-processed images, the better. In-camera noise reduction, done using dark frame subtraction, may also be a useful feature to have but better noise reduction can usually be done externally by taking and applying multiple dark frames (see below). Some cameras have the capability to do other types of in-camera image enhancement (e.g., focus sharpening, contrast enhancement, etc.). While these other features are helpful for general snapshot photographers, for astronomical imaging it is useful to be able to turn these features off. As with compression, image enhancement can be done with software later, but if you plan on doing any kind of image stacking/averaging/subtraction, it is best to do these on raw images that have not been image processed in the camera.
In the lowest price range, cameras that cost under $50 or so will be extremely limited. If you already have one, feel free to play around with it. But if you are considering buying a camera because you have an interest in getting into astronomical imaging, consider investing a bit more if you want to be at all satisfied with your results. Going up to $100-$200, the capabilities of the camera will very likely constrain your range of targets, but you should be able to get decent shots of the moon.
In the range of $300-$600 dollars, you should be able to get a very versatile camera. Two product lines that are popular among members of the digital_astro Yahoo! Group are the Nikon Coolpix line (8x5 and 9x5 series as well as the newer 7x00 and 8x00 series, where the "x"es represent digits) and the Olympus x040 line of cameras. Though these are popular lines, other lines and brands in this price range make fine choices as well. Your ideal choice will depend on your particular needs. Searching the last six months of digital_astro Yahoo! Group message archive is highly recommended, as you will surely be able to find previous threads for advice on camera purchasing by choosing appropriate search keywords (e.g. advice, recommendations, purchasing, buying, and so on). Finally, at the upper end of this price range, consider getting a used Canon or Nikon DSLR. If you are careful and make sure that all of the required functionality is there, you can start doing prime focus DSLR imaging at the upper end of this price range.
Just above this price range sit the Canon Digital Rebels, the 300D and the 350XT. Used 300D's can be obtained on forums such as Astromart for around $600-$750 and these will provide fine images. New 300D's (these are out of production now) can be obtained for around $750 while the new 350XT's are priced at around $1000.
Among the near-professional grade cameras, the most talked about models are the Nikon D70 and the Canon 20D. These two can do very long bulb exposures, are extremely capable, but will set you back in the neighborhood of $1,500. For an investment on that level, you should have some professional use for the camera, or be sure that digital astrophotography is going to keep your interest for a while. These cameras will take the best images however. Keep in mind that all of the DSLR's mentioned till now have image sensors that are smaller than a true 35 mm film camera (by a factor or about 0.625). Professional grade cameras can be found that eliminate this restriction (e.g., the Canon 1DS Mark II's) but these typically cost more than $5000.
As indicated previously, which camera is best for you in particular depends on a variety of things, not the least of which is what you may consider desirable in a camera for from a non-astronomical use perspective. The short answer is that there is that no one camera is the best choice for everyone. Hopefully the general guidance provided here, along with the information in the previous discussion of features, should give you a starting point.
The DSLR and point and shoot cameras mentioned have all been designed for terrestrial photography. They contain a sensor array and a series of red, green, and blue filters mounted to the sensor chip in what is called a Bayer array. In addition to these filters, all of these camera include a filter that will block the Infrared light that the camera is exposed to. This latter filter (sometimes called a blue meanie with apologies to the Beatles) will also block out a large amount of the energy in the red wavelengths. Many nebulae have much of there energy in this wavelength and the blockage of same will impede the camera's ability to image in these wavelengths.
In order to enhance the sensitivity of these cameras, a secondary market (and, frequently, of talented amateurs) has arisen that will remove the filters from the Canon 20D's, the Canon 300D's, and the Nikon D70's. Some success has also been reported with respect to the Canon 10D. Finally, Canon has released a Canon 20Da that is built without the IR filter and that has some additional modifications of particular interest to astrophotographers. The Canon 20Da costs a bit more than $2000.
Removal of the filter will typically cost about $500 more but the improved sensitivity will more than compensate for that. These cameras can also be used for daytime photography if careful attention is paid to the white balance characteristics (again, see below).
Equipment that you should take into account when budgeting for your camera include rechargeable batteries and a charger, a mount or adapter for coupling your digital camera to your scope, additional memory cards for your camera, and a remote control unit (and/or computer cables). In doing research for my equipment purchasing, I found thomas-distributing.com to be a good source for rechargeable batteries and chargers, and newegg.com to be a source for inexpensive memory cards.
As mentioned earlier, a remote control is a very useful accessory if one is available for your camera. Use of a remote control can be more convenient than manual operation, and avoids vibrations from manual operation of a camera. Some cameras can be controlled from a laptop, so if you already own a computer laptop, you may want to look into this as well. Some people whose cameras did not support remote control or whose remote controls were limited have gone and jury-rigged their own hands-off imaging devices (such as a bracket that clamps onto a camera and allows a screw to be tightened to press and hold down the shutter button).
Other convenient but non-essential accessories that you may want to consider include an AC adapter and a memory card reader for your computer. Even if you plan on purchasing an AC adapter, you should still invest in a set of rechargeable batteries and a charger, for non-astronomical usage if nothing else. Digital cameras can go through batteries quite quickly (especially when the temperature is low). Relying on disposable batteries can become quite an expense, whereas a rechargeable battery that costs less than $3 can be recharged hundreds of times. (In addition to being more expensive, use of disposable batteries needlessly adds environmentally unfriendly chemicals and materials to landfills.)
While many of the images that are seen are taken through a telescope using either prime focus, afocal, or eyepiece projection techniques, there are a class of large objects (e.g., the Andromeda nebula, the North American nebula, the Pleiades, wide field milky way images, etc.) for which piggyback astrophotography is appropriate. In piggyback astrophotography, the camera is mounted on a tracking mount (or on a telescope on a tracking mount) and a telephoto lens is used to directly image the sky. Depending on they type of lens used, images taken this way can rival those taken through the smaller telescopes.
Perhaps one of the most important items that will be needed for imaging (at least as important as the camera) is the mount which will carry the telescope and the camera. This mount needs to accurately track across the sky and counter the motion of the earth. If you are imaging at resolutions of a few arc seconds per pixel and taking images over minutes of time, the mount needs to be accurate to fractions of a degree per hour. Furthermore, it must be sufficiently massive and well made so that vibrations (caused by the wind for example) do not cause it to move in undesired ways.
Many of the people who are seriously into imaging of the dimmer objects are using German Equatorial Mounts (GEM) such as the Losmandy G8, G11, and Titan; the Vixen GEM; the Astrophysics Apxx00 series mounts; or the Takahashi mounts. These mounts provide significant amounts of stability, superior tracking accuracy, polar alignment assist mechanisms, and the ability to correct for Periodic Errors. Fork mounts such as those offered by Meade can also be made to work quite satisfactorily if they are mounted on a equatorial wedge.
While almost any mount will suffice if one is just imaging the moon and a rigid mount will suffice for the planets (since the imaging intervals are still short), one must graduate to a much more stable mount when imaging the Deep Space Objects.
In addition to a stable mount, one must decide whether or not one needs a "GoTo" mount. The distinct advantage to a "GoTo" mount is that when one decides on the object to be observed/imaged next, he or she can just select it using a menu and then tell the computer built into the mount to "GoTo" that object. If one is imaging the dimmer objects, this can be quite valuable since these objects can likely not be seen in the images until they are processed.
North American Piggy Back Nebula Image © Teri Smoot. Used with permission.
For DSLR's, the standard method is to use a T-ring and T-adapter. These will screw into the front end of your camera just as though they were a lens and then fit directly into the eyepiece mount. T-rings and T-adapters typically do not include any optical elements. They can also be found in both of the typical eyepiece diameters (1.25" and 2").
For the Point and Shoot cameras, some people have built their own perfectly functional adapters from scratch using parts you can get at a home improvement store, while others have assembled higher-precision adapters from off-the-shelf components, including standard (non-digital) camera adapters and step up/down rings. Links to a variety of homegrown digital camera adapters that have been designed by different people can be found on the Homegrown Digital Camera/Telescope Adapter Page.
In addition to homegrown adapters, there are also many different commercial adapters on the market, made expressly for connecting digital cameras to telescopes. There is somewhat of a spread in cost among the commercial products, and there is by no means a clear-cut answer to which one is best. All of them have advantages and disadvantages: some are easier to connect and disconnect than others, some limit the types of eyepiece that can be used with them more than others, achieving proper alignment is easier with some than with others. Further more, not every adapter can be used with any digital camera and/or with any scope. In other words, your equipment (camera, scope, and eyepieces) may affect which ones you are able to use in the first place, and from that set which ones may be best for you. A more complete summary of the various alternatives is too long to include here, but can be found on the Digital Camera/Telescope Adapter Page, which summarizes equipment limitations, advantages and disadvantages, and provides links to vendors. Also, the company Scopetronix has a very wide list of adapters for most every camera and discussions as to how to use these adapters.
Some of the adapters are made to connect to specific cameras, but most of them are generic ones which use a standard T-thread connection. Since the point and shoot digital cameras do not have standard T-threads, you will generally need a step-up or step-down ring to go from your camera's thread to a T-thread in order to connect your camera to a generic adapter. In some cases, depending on the design of your camera, you may also need an extender of some sort. Links to several online sources for step up/down rings, extenders, and related items, can be found at the bottom of either of the two adapters pages mentioned above.
Once you have decided which camera(s) you are interested, you should compare prices for the camera you are interested in. You can compare prices at online retailers using any of a number of price comparison web sites (such as pricegrabber.com, shopper.com, dealtime.com, pricescan.com, and many others). There's quite a bit of overlap in the retailers they cover, but their coverage isn't identical so it doesn't hurt to try a couple. You may be tempted to try buying at a local brick-and-mortar store. Feel free to shop around, but in my experience I found that the costs at some online retailers were as much as 40% less than places like Walmart, whose prices are already discounted below suggested retail prices.
On the other hand, you should resist the temptation to jump at the lowest price you find. Several online camera retailers run shady business practices. Things I read about while doing research for my purchase include misrepresentation, bait-and-switch tactics, and canceling orders for customers who were not willing to buy overpriced accessory packages. Since accessory packages are a way to make up for reduced profits on deeply discounted cameras, there's nothing wrong with dealers offering these packages. But I've heard of dealers delaying shipment or outright canceling orders for people who declined to purchase additional items. If you decide to buy a camera online, I would recommend checking resellerratings.com, which provides ratings (both numerical scores and comments from customers describing their experiences) for tons of online retailers.
One additional shady practice you should look out for is the selling of gray market or refurbished items without identifying them as such. Refurbished items are not new and should be identified as refurbs. Gray market items are items that are packaged for sale overseas and somehow find their way into inventories of dealers in the US. The most serious problem with gray market cameras is that manufacturers generally do not honor warrantees on these items.
Finally, since the market for digital cameras is changing very fast, you should also consider buying a used camera. Frequently, you can find someone who has graduated from a very satisfactory Canon 300D or 10D to the new 350D's or 20D's and purchase their camera at a price that is significantly less than what you would pay for a new one. These cameras frequently are nearly as good as new cameras and will take very good astrophotographs. While E-bay and other such sites are possibilities, you should also check out the offerings on Astromart or from other astronomy businesses.
Image © Daniel Ethier. Used with permission.
Heat generates noise in CCD (charge coupled device) chips. Many deep sky objects require long exposures, during which noise can easily exceed the signal you are trying to capture. CCD cameras for astronomical imaging use Peltier devices (solid-state heat pumps) to cool CCD chips to temperatures at which they can be operated for long-exposure imaging with minimal noise. There are other benefits to how CCD cameras are designed. For example, they can have more sophisticated electronics with functionality geared specifically toward astronomical imaging (e.g. anti-blooming gates, pixel binning), and more robust designs that can better withstand exposure to temperature extremes and humidity.
People using CCD's are generating some of the best current astronomical images. These images are generated by taking multiple exposures with different filters for the Luminance, the Red, the Green, and the Blue channels and then combining them in post processing. CCD images usually consume more exposure time as well. However, the secret to getting a good astronomical image from a Digital Camera is to also grab as many photons as you can.
Not all CCD cameras are that expensive. The low range astronomical CCD cameras are less expensive than a good midrange (not even professional grade) digital camera. And if you are a do-it-yourselfer, you can save some money by building your own CCD camera for even less money (see the Cookbook CCD Camera FAQ at http://www.wvi.com/~rberry/cb245faq.htm for more information and pointers to other resources). But the Peltier device is not the only thing that results in higher costs for commercial astronomical CCD cameras. The specialized electronics and more robust designs mentioned in the previous question contribute to the cost, as does the fact that the CCD camera makers don't have the economy of scales that makers of digital cameras (which are sold by the millions) benefit from.
You can! Some people have been quite successful at doing basic imaging with webcams to produce relatively good quality results. In fact, some of the better planetary images that are now being taken are being taken with webcams. However, because the better digital cameras come with features that are not available on webcams, the best that can be achieved with digital cameras exceeds the best that can be done with webcams (at least for items other than planets -- webcams regularly take 1000's of images in a few minutes and then use the best few for their images).
This FAQ is written for the digital astro group and focuses on digital cameras because that's what the group's emphasis is, but some portions of this FAQ (such as the image processing section) apply equally well to webcam imaging. If you are interested in webcam imaging, check out the QuickCam and Unconventional Imaging Astronomy Group (QCUIAG), which has a membership approaching 1000 people (see their web page at http://www.astrabio.demon.co.uk/QCUIAG/.
Typically, digital video cameras give the user less control over imaging parameters (aperture, shutter speed, etc.). The more common video formats also do more image compression than some of the still image formats, which results in less detail and unwanted compression artifacts. In addition, digital video cameras often don't provide options for formats or compression. That having been said, people are managing to get impressive results for certain types of imaging using digital video cameras. And of course, with some digital cameras able to take short movies and some digital video cameras able to take more traditional still images, commercial products are starting to populate the previously empty area between the two technologies. You can learn more about video astrophotography by joining the videoastro Yahoo! Group. But again, the focus of this web site is biased by my own activities in the use of digital cameras.
In recent months, there have been announcements in the astronomy magazines about new low cost, easy to use imaging devices from the major telescope manufacturers. One example of these is the Meade Deep Space Imager (DSI). This device is fairly new and there is not a lot of experience with it to date. However, it appears to be a simplified version of a CCD. With proper use, it seems that it will generate acceptable images however, these do not appear to be of the same quality as those generated by the current crop of DSLR's.
Image © Ginger Mayfield. Used with permission.
It's certainly easy to start experimenting simply using a hand-held camera. However, in doing so you may soon find yourself running into several limitations. The first limitation is basic ergonomics. It takes two hands to hold your camera over the eyepiece reasonably well. That's not a problem for snapping shots, but you want to do imaging often, and you like to switch among different eyepieces, you will find that the basic setup task of focusing, which requires holding the camera over the eyepiece and turning the focusing knob on your scope at the same time, becomes a chore.
The second issue you run into is exposure time. Although the slight motion of your hands is not a problem when taking 1/500 second exposures of a full moon, if you want to try your hand at taking longer exposures of dim targets, you won't be able to do it with a hand-held camera.
With a hand-held camera you also run into alignment problems. If the optical axis of the camera is not parallel with the optical axis of the scope at the eyepiece, the focal plane will intersect the CCD chip in the camera at an angle rather than falling flat on the chip. Depending on the severity of misalignment, this can cause uneven focus with some regions of the image being in focus (where the focal plane crosses the CCD chip) and increasingly out of focus moving away from that line (where the focal plane is in front of or behind the CCD chip).
Lastly, if you have a motorized scope or equatorial platform that gives you tracking capability, with a hand-held camera you lose the advantages that tracking gives you. A hand-held camera won't adequately track the scope's motion even if the scope is effective in tracking the sky's (apparent) motion. One benefit of tracking is the ability to take longer exposures without blurring due to motion of your target in the field of view (the Earth's rotation), but even for short exposures tracking has the advantage of being able to take multiple images of a target in the same position, which is useful for certain types of image processing (e.g. image stacking and image averaging).
One alternative to a hand-held camera is a tripod-mounted camera, where a tripod is used to hold the camera at the eyepiece. This is definitely better than a hand-held camera, because it allows longer exposures because the camera is not moving in your hands. However, unless you have an equatorial platform on which the tripod can sit, motorized tracking will not be possible so the length of exposures will still be limited due to the motion of objects in your field of view (the Earth's rotation again). In addition, with a tripod the alignment of camera and scope optical axes remains difficult and needs to be repeated each time you reorient your scope to point at a new target.
If you want to make a hobby of digital camera astrophotography, it really does make the most sense to use some kind of camera adapter to mount your digital camera to the scope. See the Which adapter... question in under the Digital Camera Equipment questions for more specific information.
The type of focusing you do will depend on the type of camera and how you are using it. For Point and Shoot cameras, a good way to focus the camera is as follows: (1) if possible with your camera, manually set your camera focus to infinity (or possibly set to macro if your camera has a macro mode, as discussed in the previous question), (2) zoom in as far as you can using the optical zoom, as well as a digital zoom if your camera has such feature, (3) point the scope at a bright star, the more overhead the better to reduce atmospheric effects on the image, (4) connect your camera to your telescope, (5) adjust the focuser of your scope so that the image of the star is as small as possible, (6) if you used digital zoom for focusing, remember to turn it off before doing your imaging. If the moon is visible and not too low in the sky, it also makes a good focusing target.
Note however, that with either of these methods, the low resolution of LCD viewfinders can sometimes make it hard to know when you have the best focus. An image may appear sharp in the camera's LCD display but may appear unfocused when viewed at full resolution. What has happened is not that the full-resolution image got worse, it's that the reduced-resolution image in the LCD display appeared to sharper than it really was.
Here are some additional tips that can make it easier to achieve focus using the above procedure:
1. With some cameras, even when the conditions warrant a fully open aperture, the aperture remains at a default position and does not fully open unless the camera's button is pushed down part way. A not-fully-opened aperture can make a target star appear dimmer on the camera display. In these cases, holding down the button will open the aperture and brighten the image on the display, making it easier to focus using a bright star.
2. Increase your camera's sensitivity by using the highest ISO setting available for focusing purposes. Again, with some cameras you may need to hold the camera's button down part way to make this setting change "active" and brighten the image you see. In most cases, you will not want to use the highest ISO level for imaging, so remember to switch it back when you are done focusing.
If your digital camera has a video out jack, any of a number of external viewing devices will make it easier to know when you have achieved the best focus. Among the things that people have used are external monitors or small portable televisions. Small LCD screens in the range of 5" to 5.4" sold as portable displays for home video game systems such as the Sony Playstation or Nintendo Gamecube cost more, but are preferred by some due to their more compact size. The InterAct Mobile Monitor is a popular model among digital_astro users. Most external devices that have a video in jack will give you more resolution than your camera's LCD viewfinder. Finally, Marshall makes a monitor that is very popular with several of the people on the Digital Astro forum. It has the advantage that is provides a higher resolution than the Nintendo types of monitors.
It is also possible to buy or make a Hartmann mask to help with focusing. A brief article on Hartmann masks can be found here. Ron Wodaski, author of The New CCD Astronomy provides a nice discussion of Hartmann masks in an online sample chapter of his book, available at http://www.newastro.com/newastro/samples.asp.
If your camera has a TTL optical viewfinder (a viewfinder that looks through the lens of the camera), focusing using the optical viewfinder can provide better focus than using the LCD viewfinder because of the LCD resolution issue mentioned above. This assumes however, that you have good vision or are wearing glasses or contacts that provide good corrected vision. If you have poor or uncorrected vision, when you focus using a TTL viewfinder, the best focus for your eyes will compensate for your imperfect vision, and the image obtained by the camera will actually not be at its best focus. If your camera does not have a TTL optical viewfinder, the viewfinder won't be of much help in focusing since looking through it will give you a nice view of the outside of your scope. The advantage of the methods described in the earlier paragraphs is that they are not dependent on good or corrected vision in order to obtain good results.
Once focused with a given eyepiece, the focus of your camera should not be very significantly affected by switching targets or by zooming in or out on a target, though periodic tweaks can help. The camera generally will, however, have to be refocused after changing eyepieces.
DSLR's offer a bit of a different challenge when it comes to focusing. Unlike Point & Shoot cameras, you rarely will see a live image on the LCD (or video out) during focusing (the recently releasec Canon 20Da does overcome this limitation). This is because the light path through the camera to the focuser bounces off a mirror covering the sensor during the focusing mode. When you are ready to shoot the image, this mirror moves up to uncover the image sensor.
There are several different methods that are used to focus a DSLR. First, and, perhaps the easiest, is to focus on a very bright object (i.e.,) the moon using the telescope's focus knob. Then you move to the object you want to image and take the images without changing focus at all. This will work reasonably well if the object you are looking for is visible in the viewfinder (unlikely) and is not too far away from the moon (so you don't get gravity induced changes in the optics). As can be seen from the above, this method has inherent difficulties.
The second method involves using a magnifying optic over the camera's viewfinder. For Canon cameras, this viewfinder is called an Angle Finder C. This method works very well but one must be very careful to get the diopter of the viewfinder and of the angle finder set accurately.
The third method uses a focusing device such as those made by Stilleto. This is a device that replaces the camera (at prime focus) and works like a knife edge. After the correct focus position is found, the focusing device is removed and the camera is put in place. If all is stable, the system is then in focus.
The last method depends on a laptop and a set of focusing software. The two most common programs that allow this are called ImagesPlus and DSLRfocus. With either of these, an image is taken and displayed on the laptop's screen. The telescope focus is then adjusted and another image is taken. This step by step process is iterated until the best focus is gained. At that time, the camera is put into a mode where the final images are taken (with the desired ISO, image format, exposure length, etc.). The software used in this method will also likely calculate parameters helpful in getting good focus.
If you are using a Point and Shoot camera or doing piggyback astrophotography with a DSLR, you should, in theory, set the camera's focus to infinity. If you are doing afocal imaging, although the eyepiece is right in front of the camera, the target as seen through the various lenses appears to be at infinity. When you observe visually, you focus not on the surface of the eyepiece but on the distant target whose light is being focused by the telescope's optics and your eye's lens; The camera works the same way.
Most of the people who have tried doing tests to compare focus at infinty vs. macro mode have obtained the expected result (better images with focus at infinity). However, a number of people have reported better results by setting using macro mode instead. Given what "correct" result should be, Steve dug around a bit online to find out why the opposite might be true in some cases.
It turns out that some cameras, most notably some of the Coolpix models, do not focus well at infinity when the camera is at full zoom (as is usually the case when doing astronomical imaging). At full zoom, proper focus at infinity is achieved by manually setting the focus at around 30 ft. -- the infinity setting moves the lens too far. In other cameras, including a couple of the earlier Olympus models, the infinity setting was not calibrated correctly. Proper focus at infinity was achieved with focus set at less than infinity, regardless of zoom level.
Since the camera's electronic controls determine how the lenses move under various settings, such flaws can be addressed through software. In at least some of these cases, the focusing issues were recognized by the manufacturer because people reported that upgrading the camera's firmware to the latest version fixed the problem.
So in other words, there is no one answer to this question that will apply in all cases. Setting focus at infinity should give the best results, but it may not in all cases. The first step should be to upgrade your camera to the latest available version of the firmware if firmware upgrades are possible. (See your manual for instructions on how to check the current version, and how to perform upgrades.) If you are inclined to to a comparison yourself, try both ways and see which gives better results... if it's macro mode, use that instead. For any tests, be sure to use a target where you can reasonably expect the focus of an image to be decent enough to assess (e.g., the moon, or a distant target during the daytime, rather than a planet or a DSO).
There are several reasons your camera may have difficulty achieving good focus. First of all, as indicated by the focusing procedure outlined above, focusing of the scope must be done while viewing a target through the camera. If you focus the scope while visually looking through the eyepiece and then attach the camera, it won't be properly focused because the lens of your eye and that of the camera are different.
Next, if it isn't obvious, what either you or your camera sees through the scope is limited by the quality of the image that the scope produces. Your camera should be able to focus on a target that appears sharp through the scope, but it will not be able to unblur the image of a target that is degraded due to bad seeing, tube currents, or overly high magnification.
There are several other possible reasons your camera may not be focusing correctly. First off, with autofocus turned on, there are a couple of reasons why the camera may not focus correctly. One reason is that cameras which use a sensor mounted on the body of the camera for focus rather than TTL (through the lens) focus will focus the body of the telescope a few inches in front of the camera, rather than the target that the scope is pointed at. Even cameras that do do TTL focusing can sometimes have difficulty locking in correctly when the target is small or dim. Therefore, if your camera has manual focus, you should manually set the focus at infinity (or possibly set to macro if your camera has a macro mode, as discussed in the previous question).
Another potential issue concerns the method for connecting your camera to your scope. Several of the available methods consist of an adapter that gets inserted in the scope's focuser. Eyepieces are inserted into this adapter, and the camera is connected to the adapter after that. Because eyepieces are inserted in the adapter rather than in the focuser itself, this has the effect of moving the eyepiece farther back along the optical path. Depending on the geometry of your particular scope and the range of travel of the focuser, you may find that you are simply not be able to focus the scope with certain eyepieces because of insufficient focuser travel. A possible remedy for some eyepieces may be to use a Barlow lens (if your mounting method allows this) inserted as usual between the focuser and the eyepiece. A more involved workaround is to move the mirror cell of the scope forward toward the front of the scope (again if your equipment allows for this), thereby moving the focal point back along the optical axis, hopefully to a point the focuser can reach. If you run into this issue, depending on the severity you may end up having to do without certain eyepieces, or simply finding another method for connecting your camera to the scope.
Lastly, you may find that you are able to reach a relatively sharp focus with your camera using the previously described procedure, but that when you view your images on your computer later you find that some portions of the image are more sharply focused than others. Assuming the optics themselves are not the problem, this is probably caused by an optical axis alignment problem, i.e., the optical axis of the telescope is not aligned with the optical axis of the camera. Although camera mountings where a threaded camera barrel is attached to the telescope with adapters and step up/down rings ensure a good axial alignment, good alignment is harder to ensure with adapters designed for cameras that do not have threaded barrels, and even harder still with a tripod-mounted or hand-held camera. If your camera is mounted via a threaded barrel and you have repeatedly non-uniformly focused regions in your images, it be a problem with the optical axis of the scope itself, such as an unsquared focuser.
Many of the objects that we try to image tend to be very dim. When we look through the viewfinder, it can be very hard (or impossible) to see the object we are trying to image and to get it positioned correctly in the field of view. There are only a few ways to overcome this. First, if you are using either a laptop and/or the magnifier for the finder, then the process used for focus can also be used for composing the image. Here, the image is taken to focus and then the whole telescope is moved in Right Ascension and/or Declination to center the image.
Alternately, we can use a planetarium program and decide what Right Ascension and Declination we are trying to center on. A GoTo mount can be used to "GoTo" that location or one can find a bright object near the desired location and then use the mounts Right Ascension and Declination circles to move the appropriate delta RA and delta dec to center the object.
In general, a higher ISO value equates to higher sensitivity to light. With regular film, this is done by modifying the light-detecting medium (the film) itself, which results in grainier images. In digital cameras, the light-detecting is done by the CCD chip, and that doesn't get changed when you change your ISO setting. The increased sensitivity to light is obtained by increasing the gain on the CCD sensors. This amplifies the signal from incoming light, but also amplifies unwanted signals due to noise.
Most people tend to stick with the lowest available ISO setting. However, for dimmer targets, it may make sense to at least try higher settings. Between newer CCD chips which are less sensitive to noise, and in-camera dark frame subtraction (or external dark frame subtraction), many people find that they can use higher settings and still obtain acceptable images. The higher ISO settings can also allow for shorter imaging times and this will lessen the amount of smearing of the image on the imaging sensor.
All of the digital cameras (point and shoot or DSLR) use red, green, and blue imaging sensors. In order for a pleasing picture to be generated, the data from these individual sensors need to be assembled with the proper emphasis to each color. The white balance roughly denotes the amount of energy sensed in each of the color bands. It is the property that makes images taken under fluorescent lighting look bluish relative to images taken out of doors.
Most of the modern cameras have white balances that can be set individually or have an automatic white balance. As with other features, the cameras that allow for a manual setting of white balance are to be preferred. And, for most images, the best results will be obtained if the images are taken with a daylight white balance. For DSLR's and cameras that allow it, images can be taken in what is called a RAW format and then white balance adjustments can be made later.
A detailed treatment of noise in digital cameras is beyond the scope of this FAQ. Rather than defining all types and sources of noise here, I will simply use two general categories of random noise and non-random noise. Random noise is noise that may be different in every image. One source of random noise is cosmic rays. There is not a whole lot you can do to reduce cosmic rays. Another source of random noise is random electron motion. Since random electron motion increases with temperature, one way to reduce noise is to reduce temperature. You don't generally have control over ambient conditions outdoors, but one thing that can make a significant difference for longer exposures is to turn off the LCD viewfinder as much as possible during imaging (if your camera allows you to turn it off, that is).
Non-random noise has some level of repeatability among images. One source of non-random noise is variations in sensitivity of individual sensors (pixels) on your camera's CCD chip. Ideally, all pixels are identical, but as with any manufactured artifacts, the output of a manufacturing process is not 100% uniform but has some statistical distribution about a target. A sensor whose sensitivity happens to be particularly far from the mean will either be much more (or much less sensitive) to light, and will always appear brighter (or darker) than equally-illuminated neighboring pixels. When this problem becomes severe enough, you can end up with hot pixels, stuck pixels, or dead pixels (see this article or the Digital Astro site archives if you want to know more about these). Even less extreme variations in sensitivity can cause repeatable variations in your images.
There is not much you can do to control this kind of non-random noise, since it's essentially built into the CCD chip in your camera. There are other kinds of non-random noise that you can control. For instance, if your camera-to-telescope coupling does not completely enclose the camera lens barrel, light can reach the lens from the sides of the camera rather than only through the eyepiece as is desired. Any outdoor lighting getting in this way can cause noise, but if ambient lighting is not uniform (for example if there is a street lamp nearby), the effect of that noise on the image will not be uniform, making it more noticeable. Shielding a camera whose lens barrel is not enclosed by the scope mounting is an effective way of dealing with this type of noise.
Another tip for avoiding noise has to do with ISO settings. If your camera has multiple ISO settings, be aware that higher ISO settings increase noise. This is because a higher ISO setting is achieved by increasing the gain on the CCD sensors. This amplifies the signal from incoming light, but also amplifies unwanted signals due to noise. If you have control over ISO settings, stick with the lowest one as much as possible (though depending on how dim your target is, you may sometimes want to experiment with higher settings; and, as has been mentioned above, the use of higher ISO's may allow for shorter imaging times and lessen the sensitivity to tracking errors).
If you are a do-it-yourselfer, you can also try active cooling of your camera. Things that people have tried include cold packs of various sorts, active air cooling using a fan mounted to the camera, and peltier coolers as attempted here and here, for example. You can search the digital_astro Yahoo! Group message archives for more information on cooling methods.
You can also do things to eliminate the noise that you do have using various image processing techniques. To summarize, random noise can be dealt with image averaging and image stacking techniques. Non-random noise can be dealt with various frame subtraction techniques (e.g., dark frame subtraction). Non-random noise can be dealt with very effectively depending on how repeatable it is: the more repeatable, the more easily it can be eliminated with image processing.
If you do plan on doing any image processing, there are a couple of things to keep in mind that will provide you with images that better lend themselves to image processing later on. These include (1) use high image quality settings (very little or no compression, though no compression can lead to images with very large file sizes), (2) other than in-camera noise reduction using dark frame subtraction, turn off any options that cause image processing to be done in the camera (such as in-camera digital sharpening). The reason for this is that you can always do equivalent image processing later using software, but there are some processing techniques that you may want to perform first that will work better on unprocessed images.
Discussions of using laptops in digital astrophotography can often lead to very intense disagreements. The purists argue that anything that comes between you and the stars eliminates important parts of the hobby. And, it is true that carrying a laptop into the field means that there is one more item that needs to be powered, one more item to set up, more cables for people to fall over in the dark, a good source of unwanted light, another high value item that can be lost or broken, etc. And, very good images can be taken using camera finder magnifiers such as Canon's Angle Finder C, remote control timers, etc.
The counter argument is that most of the images taken for digital astrophotography will end up in a computer anyway. And, the computer can be very readily used for focusing, camera control, image composition, and image storage. The computer can also be used to manage the observing session using a planetarium program and to control the mount. If it has sufficient power, it can even be used for image processing and imaging sessions at the same time. People have even used their laptop as a remote control station and done their imaging in the comfort of their home while the telescope slaves away outside (or even miles away).
As with everything in this hobby, there is no one correct answer to the use of a laptop. For some, the added item will be an incredible inconvenience while, for others, imaging without the use of a laptop for camera control and focusing would be inconceivable.
If one is imaging the moon (or the sun) or the brighter planets, the exposure times are very short. Further, these objects are sufficiently bright so that backgrounds will either not be noticeable or will be easily processed out. For that reason, it will not be as critical to travel to a true dark sky site if you want to image this type of object.
The issue becomes quite different if the desire is to image very low surface brightness objects like galaxies or diffuse nebula. Here, the challenge will be to bring the low levels of illumination that come from the galaxy above the levels of the background illumination. If you are visually observing, about the only way that this can be done is to reduce the background illumination as much as possible (and increase the amount of light from the object) so that the contrast is high enough to see the dim objects. Visually, this is only possible if one decreases the background (a dark sky site) and increases the illumination by using larger telescopes.
For digital astrophotography, the issue is somewhat different. Here, when one takes an image, they get the sum of the background and the illumination of the desired object. If the background has a consistent character (i.e., is constant or has a clean gradient), then a copy of this background can be generated and subtracted from the image. The remaining light that is left is that of the object desired.
This process does not work as cleanly as described above. However, it does illustrate that a dark sky site is not as critical to the astrophotographer as it is to the visual astronomer.
The brightness levels of the objects that are imaged span a very large dynamic range (and the characteristics of the cameras and optics used to do the imaging also span a large range). Therefore there is not one simple answer to this question. Broadly speaking, however, the images of interest range from the moon where exposures of hundredths of a second are likely to be best, to the bright planets (magnitudes zero and smaller). Here exposures are frequently tenths of a second.
The next set of objects will tend to be the dimmer planets, open and globular clusters, multiple star systems and asterisms, etc. In all of these, one is imaging the stars themselves. These are point objects (or almost so) and can be dim (especially for the more obscure clusters). However, here one typically takes exposures of tens of seconds.
The surface brightness of nebulas also span a broad dynamic range. Furthermore, one frequently wants to image the bright dense portions of the nebula as well as the dimmer outer areas (see, for example, the M42 image above). For these objects, one will typically take groups of images with exposures of both tens of seconds (to get the bright dense regions without overexposure) and of hundreds of seconds (to image the dimmer regions). These sets of exposures are then combined via image processing to show the entire dynamic range of the object.
The last group of objects consists of galaxies. These objects are, inherently low surface brightness objects and the exposure times of same tend to be hundreds of seconds.
With all of these objects, multiple exposures are taken and then combined to produce the final image. The total exposure times of the best images tend to be at least an hour (depending on the camera and the imaging train). This total exposure time is the sum of the individual exposures.
A good general introduction to digital camera astrophotography can be found in Gregory Pruden's Getting Started Guide (in PDF format -- requires the free Adobe Acrobat Reader to view). Information about how to use the various features commonly found in digital cameras can be found in a document on Setting Digital Cameras for Astronomical Imaging. Without getting into specific imaging techniques, several easy things to do that will improve your images include:
1. Mount your camera well
Your ability to do good imaging with a hand-held camera will be limited. A hand-held camera is fine for short length exposures, but as exposure times increase due to decreased light (higher magnifications or dimmer targets), you will improve your results by using a mounted camera rather than a hand-held one.
2. Don't activate the shutter manually
To eliminate any vibration caused by the act of pushing the button on your camera to capture an image, don't activate the shutter manually. If your camera came with a remote control (or has one available as a separate accessory) a remote will allow you to capture an image precisely when you want to. If not, most if not all cameras come with a timer that will let you set the camera to capture an image a specified amount of time after you push the button, allowing any motion to die out during the delay.
3. Leave the LCD off
The LCD viewfinder is the main source of noise for the CCD in digital cameras, with the effect increasing significantly with exposure time. See the article on noise reduction in the articles section of this web site for more on LCD noise and noise in general. Use of the LCD will also shorten the life of the battery in the camera.
4. Take control over your exposures
If your camera allows control over shutter and/or exposure time, remember that for the same amount of light coming in, a shorter exposure with a wider aperture setting will be affected less by atmospheric effects than a longer exposure with a smaller aperture. Atmospheric motion, like any other motion, can reduce sharpness in images.
Most digital cameras come with photo editing software that has basic image processing capability. These programs generally come with basic filters that allow you to modify contrast and brightness and sharpen images. Many also include masking algorithms, such as an unsharp mask, which can be useful for improving astronomical images. Other filters, such as gradient removal filters, are used for astronomical image processing but are less common in general-purpose photo editing programs. These filters and masks can be used by people without any experience with image processing, and don't require an understanding of how the underlying algorithms work.
Another set of techniques involves the combination of multiple images to yield a single better image. These techniques include image stacking, image averaging, dark frame subtraction, and flat field corrections. Use of these techniques is generally a bit more involved, some requiring the taking of special frames for subsequent use, others requiring a bit more effort during processing for things like image registration.
A third set of more sophisticated image processing algorithms that are used for CCD imaging includes stretching, histogram equalization, and convolution/deconvolution algorithms.
A good overview for people who are new to astronomical image processing is Al Kelly's guide to Acquiring and Processing Astronomical CCD Images. This guide is officially geared toward CCD images. Though some portions of the guide are relevant to CCD equipment in particular, many of the techniques and most of the information about image processing are equally applicable to digital camera astronomical imaging.
[Notes: The lists below are intended to be representative, not comprehensive. Software is for Windows unless indicated otherwise.]
Several freeware astronomical image processing software applications are available. The most popular ones include:
Astrostack--Used mainly for stacking of images (in BMP or AVI format), though version 0.9 does provide some basic image enhancement capability. A new version (2.0) is available but image enhancement is not implemented and color images are not yet supported. The next release (2.1) is expected to be significantly improved, and will also support jpeg format.
Registax--Also does stacking of images (in jpeg, BMP or AVI format) but provides more sophisticated image processing functions. One limitation is the size of the images it can deal with (1024x1024?).
IRIS. A more fully-featured image processing application (for BMP, FITS and PIC formats), but can be unwieldy because of its interface, which is command-line oriented rather than point-and-click.
Snapp--Written by Gregory Pruden, this application still at a beta stage. A work in progress, functionality is being refined and extended on an ongoing basis. The application already does stacking of color jpegs, and provides some image analysis and enhancement capability.
CADET--An application that does not do stacking, but provides other image processing capability (for BMP and FITS formats) including advanced techniques such as deconvolution and digital development process.
iMerge--This application allows you to stack images, and also to lay out multiple images into composite or mosaic images. For the latter functionality, the software includes histogram modification functions to help match overlapping images that may have slightly different brightnesses.
Neat Image--This application builds a profile of the noise characteristics of an individual digital camera, and uses that noise profile to customize a noise filtering algorithm for your equipment. The demo version is free, and despite a few limitations (mainly in input/output formats) is almost fully functional. You might also want to check out Neat Batch, a small freeware application which adds "batch processing" ability to Neat Image. Note, the program Noise Ninja performs a similar function and is preferred by some of the digital astro group members.
Image Stacker--This application is the only image stacking application available for Mac OS (requires OS 8 or later). Because it is aimed at webcam users, it currently handles only quicktime movies and Macintosh PICT files, so you'll need some other application (such as the free iMage) to convert jpegs to PICTs. Image Stacker also provides a number of other image processing functions in addition to stacking.
BlackFrame NR-- Not aimed specifically at astronomical imaging, but a generic application that is useful for doing dark frame subtraction.
There are several other generic (not astro-oriented) image processing software applications available. These applications are much more fully-featured, with a much more diverse set of image processing and manipulation filters and algorithms available as compared to the freeware applications mentioned above. However, because these applications are not designed for astronomical imaging, they lack some basic and important (from an astronomical imaging standpoint) capabilities such as simple alignment and stacking of images.
Adobe PhotoShop--Available for both Mac OS and Windows, PhotoShop is arguably the most powerful image processing software application out there, and one of the most popular ones among the high-end users. Very diverse functionality, but at a retail cost of over $600, if you don't already own it, it probably isn't worth buying just for digital astrophotography. Note, a simpler version of this software, Photoshop Elements is frequently provided as a "freebie" when you purchase a camera. This can frequently be upgraded to a full version of Photoshop at a much more reasonable price.
Paint Shop Pro--Somewhat less fully-featured than PhotoShop, but very functional nevertheless. And at a cost of around one eighth that of PhotoShop, it's considerably more affordable.
GNU Image Manipulation Program (GIMP)--Not as ergonomic an interface as the previous two tools, but very functional. It's distributed free under the GNU General Public License so the cost can't be beat. Another nice feature is that it's available not only for Windows, but for Linux/UNIX and Mac OS X.
Lastly, there are a number of specialized image processing applications that have been designed specifically for dealing with astronomical images.
ImagesPlus--This $120 software package is probably the most popular astronomical image processing software application on the digital_astro Yahoo! Group. This is due in part to the modest cost for the degree of functionality it provides, and in part because the software is the first to really be aimed at digital camera users rather than CCD users. The author of the software is an active member of the digital_astro group, moderates his own Yahoo group, and frequently updates and extends the software based on user input.
AIP4WIN--Rather than being a stand-alone software application, this is a software package that comes as part of an book called The handbook of Astronomical Image Processing. Considering you get an informative reference resource and the software together, the price of $80 is not bad. The main disadvantage from the point of view of digital camera users is that one cannot work directly with color images. Instead, the software requires separating color images into separate RGB components, and performing image processing on the components.
Other--Other well known image processing applications include AstroArt ($149), Mira (starting at $249), MaxIm DL ($300) and CCDSoft ($349). These are geared toward users of CCD cameras, but the image processing they are capable of applies to digital camera images as well. I don't know much specific about these applications, but the range of image processing capabilities they provide runs from elementary alignment and stacking, to sophisticated processing and enhancement techniques.
Jim Solomon, one of the people who currently frequents the Digital Astro group has written an excellent "cookbook" for Astronomical Image Processing. This cookbook can be found in its entirety at http://tinyurl.com/ahyqq. Jim uses IRIS as his image processing tool and the cookbook is written with this in mind. The material in the following paragraphs is taken from Jim's cookbook (with permission) and provides an overview of how to gather and process the data. I have "genericized" the data in his cookbook to attempt to make it apply to all installations. Errors in this process are mine.
This is exclusively a "How To" for long-exposure astrophotography of Deep Space Objects (DSOs); i.e., anything requiring a very long, tracked, exposure to adequately capture. This is therefore specifically NOT a "How To" for planetary photography.
Note also that this material does not delve very deeply into the theoretical underpinnings of digital astrophotography, or even the "basics" of astrophotography. As such, it is assumed that the reader is already familiar with and understands the following concepts:
How to couple a camera to a specific telescope.
The need for a mount to "track" the apparent movement of the stars over the duration of the exposure.
The need for many entry-level mounts, and even some high-end mounts, to be "guided" to correct, in real time, tiny errors in those mounts' tracking ability.
The need to polar align the mount fairly accurately.
The basic (digital) technique of taking many relatively short sub-exposures, and then stacking them to yield a much longer effective exposure time.
The need to shoot in "RAW Mode" in your Digital SLR, as opposed to Large/Medium/Small JPG mode.
See some of the excellent Introductions to digital astrophotography available on the 'net for more info. on these topics.
The sections below describe the three phases of my astrophotography technique in detail; namely, planning, acquisition, and processing.
Here are a few terms used throughout this guide:
Frame (noun) -- synonym for exposure. E.g., if I take 15 exposures at 4min each of my target, I speak of having acquired "15 Light Frames at 4min each".
Frame (verb) -- the process of centering the target object in the imaging camera.
Lights -- frames taken with the imaging camera through the imaging scope, with the dust cap off. I.e., these are the actual exposures of the target object.
Darks -- frames taken at the same ISO, exposure time, and temperature as the Lights, but with the camera's body cap in place. Darks and Offsets are used to mitigate the effects of various noise sources in the camera.
Offsets (aka Bias) -- frames taken at the same ISO and temperature as the Lights, but with as short an exposure time as the camera allows (1/4000'th of a second in the case of the 300D) and with the camera's body cap in place.
Flats -- frames taken of an evenly illuminated target, typically the sky just after sundown. These are used to correct for vignetting in the optical path of the imaging scope, and should be taken with the lowest ISO setting the camera allows.
Target -- the DSO you are setting out to photograph.
Clipping (aka Saturating) -- the act of overloading a digital sensor to the point where it reports its maximum intensity value. A clipped/saturated frame is one taken with an ISO setting that is too high; an exposure time that is too long; or both. Clipping causes a loss of information that can never be recovered.
Many newcomers to digital astrophotography are confused by the notion of Lights, Darks, Offsets, and Flats, so here I'll give a very brief background on these concepts. The CMOS or CCD imaging chip in most Digital SLRS will very faithfully and linearly collect light from the object you're trying to image. Unfortunately, the "signal" collected from the target object will get degraded by thermal noise and other noise sources. Darks and Offsets are the means by which we try to characterize and mitigate the effects of these noise sources. Also, the telescope/lens optical train may not fully illuminate the imaging chip, depending on its size, resulting in a phenomenon called vignetting. Flats are the means by which we try to characterize and mitigate the effects of this vignetting, and further the means by which we mitigate the effects of dust that might have settled on the imaging chip in the camera.
The formula that relates these physical phenomenon, and the actual frames we'll collect over a night of imaging, are as follows:
(1) Light = (Signal * Flat Signal) + Dark + Offset
where Signal is the image of the target object we wish we could collect under ideal circumstances, and Light is the image we actually captured. Rearranging the terms, we have:
(2) Signal = (Light - Dark - Offset) / Flat Signal
But realize that the Flats we capture with the camera will, in turn, be "polluted" by Darks and Offsets in their own right, and so we must subtract Flat Darks and Flat Offsets from the Flat Lights as follows:
(3) Flat Signal = Flat Light - Flat Dark - Flat Offset
So, plugging equation (3) into equation (2), yields the final result:
Light - Dark - Offset
(4) Signal = ------------------------------------
Flat Light - Flat Dark - Flat Offset
Thus, the basic digital astrophotography technique involves capturing all of the frame types listed on the right-hand side of Equation 4, and using them in an image-processing program to produce the "Signal" term on the left. This process of subtracting Darks and Offsets, and dividing by Flats, is called "calibration" of the Lights. Note, finally, that a Dark as captured in an exposure actually contains the Offset, and, so, in processing, we will often just subtract this Dark from the Light, without explicitly subracting an Offset. But, as stated, that captured Dark actually contains the Offset, and subtracting that captured Dark has the effect of removing the Offset as well.
As will become evidently clear, setup and acquisition of data can be quite involved and, therefore, time consuming. It is therefore desireable to have as much work done "up front" as possible before going out for the night. The better the planning, the better things will go when shooting. The activities in this stage are as follows:
Pick a target. Use charting software or the many available catalogs to pick a suitable target. In particular, I try to pick large objects that fill the camera's Field of View; bright objects that have a reasonably high mean surface brightness; and objects that are well placed. Pay particular attention to when the object transits and on which side of the Meridian you'll be shooting it.
Choose a camera orientation. Determine if the object has greater extent in the East-West direction or the North-South direction, and be sure to orient the long dimension of the camera's sensor in that direction when attaching it to the imaging scope. I prefer to orient the camera in the "North is up" direction in all cases, unless the object begs for a "North is left" orientation. Examples of the latter include M81/M82, M42, and others.
If you are using a guide scope, Pick a candidate Guide Star. It will speed the process of Guide Star acquisition to have a rough idea of which Guide Star you're going to use and knowing how far, and in which direction, that Guide Star lies from the target. Also, the further the Guide Star from the center of the target, the more accurate your Polar Alignment will need to be. See the section on Polar Alignment to learn why this is the case.
Devise a plan to find the target. Do you have a GoTo mount with scary-good accuracy? Do you have Digital Setting Circles? Are you a Star-Hopping Demon? In any case, you'll need to figure out a way of centering the target in your imaging camera without removing the camera. (Why? -- because you're not allowed to remove the camera between taking your first Flat Frame and your last Light Frame.) A good way to accomplish this with a less precise mount is to use an unmistakable object as the reference object, center it, and then use a spreadsheet to compute the RA and DEC offsets to the actual target from the reading on the Hand Controller's "Get RA/DEC" function.
Take an educated guess at ISO and exposure time. This can generally be done "in the field", but if you have the time, it never hurts to research what other folks have used to shoot the same target. Or to look through your own portfolio and see what seems to work and what doesn't. I tend to lean toward 4min sub-exposures @ ISO 400, as that leaves a very large dynamic range (lack of clipping of bright objects), and 4min is usually long enough to capture a decent amount of detail in each sub-exposure. Also, at 4min, if an airplane flies through and ruins a frame, like, so what?, it's only 4min. Dimmer targets will require longer exposures and/or higher ISOs. You'll need to experiment with this to see what works best for you.
The acquisition process consists of the following distinct phases:
The setup phase can and should be done during the daytime. Basic setup of your mount, scope, laptop, etc., is way beyond the scope of this document, so I'll focus on the astrophotography-specific aspects of the setup:
0. Verify the collimation of the optics.
This isn't really specific to astrophotography, but collimation is essential to good results, particularly with the fast optics of a Newtonian. I like to orient the scope in the rough direction I'll be shooting before verifying the primary's and secondary's tilt with a laser collimator.
1. Setup and configure the Imaging Camera and Imaging Scope:
Attach the camera to the T-Ring, and insert the T-Ring into the telescope's focuser (including any coma correctors/field flatteners/Barlows/etc.). Ideally, use a 2" or larger focuser and equivalent optical elements.
Focus the camera as accurately as possible on a distant target.
Align the imaging scope's finder to the center focusing dot in the camera's viewfinder, again using a distant target.
Align the camera North-South or East-West. The easiest way to verify the orientation is to slew the mount in RA or DEC, and to make sure the object moves along the focusing dots in the viewfinder in a way consistent with your chosen camera orientation. Rotate the camera until this is as exact as possible.
Lock the focuser and make sure the tensioning knobs that hold the focuser are tight.
2. If used, Setup and configure the Guiding Camera and Guide Scope:
This section depends on whether or not you are using a guidescope and the type of guiding your are doing. See Jim's site for information on how he does things.
3. Balance the scopes in the mount:
Mount, align, and focus the cameras as per #1 and #2 above.
Remove (and replace!) any dust covers that will be off (or on) during the actual image acquisition. You want the weight distribution to be identical to how it will ultimately be when you're shooting.
Swing/slew the mount to the approximate location of the target.
Balance the scope in DEC, with a slight imbalance in the direction that opposes the motion resulting from pressing the Up arrow on the Hand Controller.
Balance the scope in RA, with a slight imbalance to the East.
When you first started taking astrophotos, you may assume that you can "blow off" Flat Fields, Dark Frames, etc. You're was wrong. Just do it. You'll thank me. There are several different methods for acquiring flats. The material below describes just one.
First, some background. The purpose of Flats is to characterize the optical train as accurately as possible, so that later, in processing, you can correct for things like vignetting (uneven illumination of the optics, particularly a darkening at the edges of the camera's field). A good set of Flats will also correct for dust spots on the camera's sensor. The ideal Flat is taken of a (perfectly) uniformly illuminated target. In practice, the twilight sky makes a reasonable approximation to this ideal, and, in particular, the twilight sky a few hours East of Zenith makes for an excellent approximation.
Thus, you can use the twilight sky (just after civilian sundown) as a "uniformly illuminated target". (Others use different methods and they may be getting better results).
In any case, here's what I do:
Do a Quick Align in the mount so you can slew to a desired RA/DEC.
Slew the scope to a DEC equal to your Latitude, and an RA that is 2 Hours East of the Meridian. I saved a User Object for this for convenience. Note that this User Object must be a "Land Object", not a "Sky Object", because, if you think about it, you want the scope to point to the exact same part of the sky regardless of the current time or date.
Take your exposures. Definitely use ISO 100 and "RAW" Quality mode. Exposure time is something you can play around with, but exposures in the 1/8 to 1 second range (at f/5) are likely appropriate. Begin snapping right around sundown, as soon as the histogram on the camera's LCD begins to move off of the saturation side of the graph, and just keep on snapping until the histogram is rather far to the left. Later you can figure out which of these frames you'll use to compute your Master Flat Frame. Right now we just want to capture a boatload of them to allow flexibility later on in processing.
Important: Make sure to move the scope East between each exposure, to ensure that any stars that are recorded can be eliminated later in processing. I move by "30 RA seconds" between each frame, which is equivalent to an angular movement of roughly 6 arc-minutes at my Latitude (remember, we set the mount's DEC to our Latitude).
When finished capturing the Lights, very carefully remove the camera from the T-ring, without moving/rotating the T-Ring, auxiliary optics, or focuser, put the body cap over the bayonet, and capture 15 Darks at the exact same settings you used to capture the Lights (e.g., 1 second exposures at ISO 100).
With the cover still in place, take 15 Offsets at the same ISO (hopefully ISO 100) and at the fastest exposure your camera allows, which is 1/4000'th of a second for the 300D.
At this point it's a good idea to remove the flash card from the camera and copy the Flats (Lights, Darks, and Offsets) to my PC ... just to make sure you don't accidentally delete them.
Remove the body cap from the camera and carefully pop the camera back onto the T-ring.
In a guided system, one need not worry about RA or DEC drift, especially if the guiding software is guiding the mount in both RA and DEC. However, Polar Misalignment causes Field Rotation in addition to RA and DEC drift, and it is this rotation that is problematic. In general, field rotation gets worse as the Polar Alignment error gets larger. It also gets worse for objects close to the poles (i.e., DECs near +90° or -90°). And it becomes more problematic the further the Guide Star is from the center of the imaging camera's field. The latter is so because the field will appear to rotate around the Guide Star, and the further it is away from the center of the imaging camera's field, the more that field will tend to "slide off the imaging camera" over the course of the night's image acquisition.
So, how accurate must the polar alignment be? Well, accurate enough for the task at hand. Certainly it must be accurate enough so that no discernible field rotation occurs over the duration of a single exposure. And it must be accurate enough to prevent the field from rotating over the course of the night to the point where there's very little intersection among all of the night's Light frames.
One way to achieve polar alignment is to Drift Align to the point where you see no discernible drift over a span of 4 to 5 minutes. Drift Alignment isn't really as scary as most people think it is. You should learn how to do it. One of the good tutorials on Drift Alignment can be found at Andy's Shotglass (click on the Drift Alignment link).
There are wonderful tools available to DSLR users to help them focus, and DSLRfocus is one such tool. Spend some time at this stage to focus as accurately as possible. You'll be glad you did. Take an extra 5min to really nail the focus -- I assure you that N perfectly focused frames will give you a better result than N+1 poorly focused ones! Here's one procedure using DSLRfocus:
Shut down your guider and disconnect its cable.
Connect the camera to the laptop via the supplied USB cable.
Run DSLRfocus in "focus mode", turn the camera on, and connect to the camera.
Center a star in the "dead center" of the field of the DSLR.
Use ISO 100 and an exposure in the range of 1 to 4 seconds. You want the star to be as bright as possible but without clipping!
Use Medium/Normal image quality at first. Then hone in and verify with Large/Fine quality.
Use mirror lockup if your camera supports it.
When you think you've got the focus nailed, engage the focus lock on your scope's focuser and then take another shot "just to make sure". If the focus isn't perfect, repeat until it is.
There are many ways to find and center the target in the field of the imaging camera. For really bright objects, one can just center them using the imaging camera's viewfinder. M42 is an example of one such target. For more illusive targets, you can use the spreadsheet discussed in the planning area above to compute the RA and DEC offsets (deltas) between the Target Object and some bright, nearby, easy-to-find, unmistakable, Reference Object. The Reference Object is almost always a nearby bright star or planet. You then center the Reference Object in the imaging camera's field and then pull up "Get RA/DEC" in the mount's Hand Controller. Then subtract the offsets from my spreadsheet, and slew the mount to the resulting coordinates. In all cases, the slews must finish with the Up and Right arrow keys, in order to deal with any backlash in a consistent fashion. In my experience, this method is amazingly accurate and easy to execute.
In any case, here's one procedure to acquire (and verify) my target:
Use the "Precise GoTo" spreadsheet as described above to get the target centered.
Still using DSLRfocus in "focus mode", take a test exposure (ISO 1600, 30 seconds, Small/Normal quality) and inspect the result. Even fairly dim targets will reveal themselves at these exposure settings. If the target is not centered correctly, slew the mount in the appropriate direction and repeat this process until it is.
Using the mount's Hand Controller, store a "User Object" (Save Sky Object) at the current coordinates, so you can easily return to this location if you accidentally hit a slew button or you accidentally change the RA or DEC when removing/replacing dust caps.
In DSLRfocus, set the camera to the ISO you'll actually use for your Light frames, to RAW quality mode, and to Bulb exposure, then disconnect from the camera and shut down DSLRfocus.
Turn off the camera, and physically remove the USB cable from the camera and the laptop.
Acquiring a Guide Star can either be simple and quick, or excruciatingly painful. The pain factor is inversely proportional to how well the Guide Scope's finder is aligned with the center of the webcam's field in the Guide Scope. If it is very well aligned, then putting the Guide Star in the finder's cross hairs will land the Guide Star on the chip every time. Otherwise, prepare to be frustrated. The sequence depends on how one does guiding and beyond the scope of this FAQ. Jim talks about what he does in his cookbook.
Consider this step the equivalent of a "dress rehearsal". This is the final verification that you've nailed the framing, focus, and exposure settings (ISO and exposure time). You can rescue many nights that would have be wasted by taking a test shot and verifying all of the above on your laptop, and making important adjustments once finding some problems. I highly recommend doing this. Here's my procedure:
Slide the Eyepiece Cover into place on the camera's viewfinder. (That's the little, rectangular, rubber piece that comes embedded within the camera's shoulder strap. For long exposures of faint targets, that cover should always be in place.)
Enable guiding if used.
Configure DSLRfocus to take a single shot of the desired exposure length.
Take the shot.
Disable guiding.
Remove the flash card, copy the test shot to your computer, and use your imaging applications to verify focus, framing, and exposure.
Replace the flash card into the camera.
Correct any problems you found in the test shot. If the signal level is too low, increase the ISO and/or increase the exposure time; if the highlights are clipped (saturated), lower the ISO and/or decrease the exposure time. You may also need to slew the mount in RA and/or DEC if the framing is off. If so, watch which way the Guide Star moves while slewing; that will allow you to easily reacquire the Guide Star. If focus is off, which it shouldn't be -- but anything's possible -- go back to the Focus section and start over.
If necessary, recenter the Guide Star (the process of popping the flash card out and back in might be enough to nudge the mount a bit).
If you made any corrections, repeat this entire process to verify that everything is correct. In everything is correct, go on to the next step.
If the last step is the dress rehearsal, then this step is the main performance. One easy way to improve performance is to dither between exposures in order to maximize the signal to noise ratio of the stacked result. Dither, means to slew the mount ever-so-slightly in RA and/or DEC in a random direction between each exposure, in order to move the target around on the imaging chip. This serves to mitigate the problem caused by uneven sensitivity of the individual pixels on the chip, as well as the effect of hot pixels. It will allow you to stretch the final processed image a bit more than you otherwise would, without revealing ugly-looking "pattern noise" in the background, and therefore allow you to reveal more detail in your target. I highly recommend it, but I acknowledge that it's labor intensive and a royal pain in the you-know-what. Controlling your laptop remotely with something like pcAnywhere can ease the pain level of dithering.
Some more comments on dithering. You must completely disable guiding while dithering; otherwise, GuideDog will return the Guide Star to exactly where it was previously, thereby negating the dithering action. To completely disable guiding, be sure to click (uncheck) the Guide button and the Lock button before slewing the mount. Then reenable guiding by clicking the Lock button, selecting the Guide Star, and clicking the Guide button again.
Here's a procedure for capturing Lights:
Verify that the Eyepiece Cover is in place on the camera's viewfinder.
Configure DSLRfocus with desired number of frames, inter-frame spacing, etc. I generally allow for 1min "down time" between frames, which gives me time to do my dithering and for guiding to stabilize before the next frame begins.
Enable guiding.
Let 'er rip!
Dither: Between each frame, disable guiding completely (both the Guide and the Lock buttons in GuideDog), slew the mount a TINY amount in a random direction, re-enable guiding, and repeat.
Shoot as many frames as you can. More frames are always better than less, and with all of this setup time, and with all of the time that you will spend processing, it makes sense to shoot as many Light frames as possible. Shoot until the object sets or the realities of your sleep requirements dictate.
A few more notes on dithering. The "pattern noise" on these consumer DSLRs tends to be vertical and horizontal in nature. So, a really bad dithering pattern would be to slew the mount in a linear fashion, either in RA or DEC. A much better dithering pattern is an "outward spiral" which, when you start to get too far away from the center, you can spiral back inward. Also note that a smidgeon of field rotation (from polar misalignment) can actually be a good thing in this regard, since it will further serve to randomize these vertical and horizontal patterns, once the frames are registered (derotated) in processing.
Now that you've acquired your Lights, it's time to (immediately) begin capturing your Darks. It is recommended that you collect at least 9 Darks, perhaps more if the Light frames had a relatively short exposure time. In any case, collect an odd number of Darks since you will more than likely be median-combining them, and the numerical median operator just likes to have an odd number of samples.
You can let DSLRfocus time the Dark frames just like with the Lights. In fact, you can go from capturing Lights to capturing Darks "without skipping a beat" in DSLRfocus. This means that if you've got 1min of inter-frame "down time" during my Lights, you can have exactly that same interval between the last Light and the first Dark, and use that same inter-frame spacing between the capture of all the Dark frames. This procedure yields an almost identical match between the Lights and the Darks in regards to the (lack of) cool-down time of the sensor between frames. Here's one procedure:
Disable guiding.
Pop the camera off of the T-ring and put the Body Cap on the bayonet.
Keep the Eyepiece Cover in place on the camera's viewfinder.
Have DSLRfocus time the capture of at least 9 Dark frames at the same ISO, same exposure time, and same inter-frame "down time" as the Lights.
Take these immediately after capturing Lights in order to make sure the temperature when taking the Darks is as close to possible to the temperature when taking the Lights.
Take at least 9 Dark frames. 15 is better.
While the Darks are being captured, I either: begin processing the Flat frames; begin putting the rest of my equipment away for the night; or begin doing visual astronomy of some of the "eye candy" that's well placed at the current time. Also continue to remind yourself that you're not done when the Darks are finished. There remains one more step ...
You're almost done. What remains is to capture a handful of short-as-possible Offset (Bias) frames. Here's one procedure:
Disconnect the exposure-timing cable that connects the laptop's parallel port to the camera's shutter control port.
Set the exposure time on the camera to 1/4000'th of a second.
Keep the Body Cap in place over the bayonet.
Keep the Eyepiece Cover in place on the camera's viewfinder.
Keep the ISO at the same setting you used for your Lights and Darks.
Take at least 9 Offsets. 15 is better.
Congratulations, you've finished your acquisition! Now it's time to process the result. Ok, actually, tomorrow morning is time to process your result. <grin>
Now that you've acquired your Flats, Lights, Darks, and Offsets, you can begin processing your frames to bring out as much detail as possible. Doing so requires some fairly sophisticated software. One application by far for this task is the freeware package named IRIS by Christian Buil. Others prefer ImagesPlus by Mike Unsold, which, as of this writing, was being sold for roughly US $200. In either case, the outline is as follows:
In what follows, only a very general outline is provided since the details of the processing are dependent on the processing package used. The details in the Cookbook are provided at Jim's website and written in terms of IRIS.
Convert RAW Flat Lights to an image processing format (CFA). It is highly recommended that all of the imaging be done in the RAW format. However, none of the image processing applications allow you to process the data in this format. You will have to convert to a common format for further processing (and, ideally, one that allows for processing in at least 16 bits).
Convert RAW Flat Darks to image processing format (CFA).
Make the Flat Master Dark. One common process involves median-combining the individual Flat Darks to make a Flat Master Dark, without using the Flat Offsets. IRIS' dark-creation procedure requires a Master Offset when creating a Master Dark and, if so,you must first produce a "dummy" offset file that contains zeros at every pixel location. The easiest way to do that is to load any of the files we've already converted to PIC, fill the image with zeroes, and then save it.
Identify hot pixels.
Select the Flat Lights. Inspect the Flat Lights one at a time looking for signs of clipping. One way to do this is pan to the brightest portion of the image, draw a rectangle with the mouse, right-click the mouse and select the Statistics item. You will then display the max, min, and other useful statistics about the selected area. Be suspicious of maximum values that are anywhere near 4095, which is the saturation value of 12-bit pixels. (But also try not to be fooled by the presence of one or more "hot pixels" that happen to lie within your rectangle. Draw a few rectangles to make sure.) I try to stay away from any Flats Light that have pixel values greater than about 3500. Generally speaking, I'll use 9 Flat Lights to create my Master Flat, starting with the brightest image in the sequence that doesn't clip and continuing through the subsequent images in the sequence until a total of 9 is reached. At this point, the easiest thing to do is create a new sequence with only those selected Flat Lights, starting with the first desired image in the sequence.
Calibrate the selected Flat Lights against the Flat Master Dark.
Create the Master Flat.
(Optional) Disk cleanup. At this point, if you need to reclaim space on the disk, you can delete all the files in your working directory except for your RAW camera files, and master flats.
Convert RAW Darks to imaging format (CFA). Use the same procedure as described in the Create Master Flat section.
Create the Master Dark. One way is to median-combine the Dark images. Be sure to save the result:
Identify hot pixels. Use the same procedure as described in the Create Master Flat section to identify hot pixels in the Master Dark. Here we're shooting for hot pixels on the order of several hundred.
(Optional) Disk cleanup. At this point, if you need to reclaim space on the disk, you can delete all the files in your IRIS working directory except for your RAW camera files, and the master flat and hot pixel maps.
Convert RAW Lights to imaging format (CFA). Use the same procedure as described in the Create Master Flat section.
Calibrate Lights against Master Flat, Master Dark, and Hot Pixel Map. For each Light frame, subtract the master-dark, divide by the flat-field, fix the hot pixels, and save the result in a new file.
Bring up the Digital Photo->Sequence CFA conversion... menu and enter the following values: Note, CFA refers to the Color Filter Array (R, G, and B) that covers the imaging sensor. This conversion from raw sensor data to RGB can also be done in the beginning of the processing.
Translate, scale, and rotate the individual calibrated lights so that the same features line up in all of them. The use of some sort of astronomical image processing software (e.g., IRIS ImagesPlus, Registack, etc.) will make this step much easier. It can also be done in general purpose image processing software such as Photoshop but this is more difficult.
Combine all of the registered data into one final image taking care not to saturate the resulting image. Again, astronomical image processing software will make this job easier.
If you didn't adjust the white balance of the original RAW images when you did the conversion, you should now adjust the white balance of the image if desired in your image processing application by appropriately weighting the red, blue, and green channels.
Inspect the edges and the four corners of the image and you'll see some "registration anomalies". The goal at this stage is to crop the image to that area which represents the intersection of all the registered frames. Adjust image window to allow you to see these registration anomalies, and to determine where to crop. By clicking in each of the four corners of the image, you can look in the Output window to see the coordinates of those locations where you clicked. Then use your image processing software to crop to the coordinates that you provide:
If there's any light pollution at your site, especially if that light pollution is uneven throughout the sky, it's likely that your image has a rather ugly-looking background gradient at this point. Also, if for some reason your Master Flat isn't correcting for vignetting, the background will also have such a gradient. Several of the image processing programs (e.g., IRIS) have a pretty good set of commands for attempting to remove this gradient, but you'll have to experiment a bit to get the desired result.
Jim absolutely loves the Hyperbolic Arc Sin (asinh) stretching function in IRIS, as he find it provides a much more pleasing result than the Digital Development Process (DDP) available in most image-processing programs. Interestingly, a variant of the asinh stretch is used by JPL to process Hubble photos. Finding the right alpha parameter (the aggressiveness of the stretch), and the right intensity parameter (a post-stretching scaling factor to prevent clipping or to make the result brighter), is largely a matter of trial and error.
Other people will use the Digital Development process or use the curves and levels function of Photoshop to perform this stretch. The key here is that the details of the objects that we are looking for will not be evident in the images made up to this step. What the stretch does is to amplify the lower level signals so that they can be seen.
If you have gotten this far, you probably have a fairly reasonable image. However, it's now time to export the data to Photoshop for final touchup. You will likely use Photoshop for some final touchup of the white balance and levels. You may also want to adjust the gamma to match different monitors, add text to the photo, rotate it to the final orientation, and, perhaps, size the image to the final production size.
Congratulations!!! You're probably now staring at a killer astrophoto.
Here are a few more "tricks of the trade" to further improve upon the results that can be accomplished above:
Bring the background level to zero before stacking to improve the effectiveness of stacking and to increase dynamic range of the normalized stack.
If you find that dark-subtraction is producing "black holes" in your frames, then create a "real" Master Offset and use that in the creation of your Master Dark. Then use "optimize"ed dark-subtraction when calibrating your Lights. The benefit will be better dark-subtraction, but at the possible expense of having more residual amp glow in the result.
Write your own program to stack, white balance, and stretch, using double-precision floating point variables. That will give you a virtually limitless dynamic range.
Archive Your Results
Burn a CD or DVD now with all of your RAWs (.crw and .thm) and your final, full-resolution, full-size processed result -- both the IRIS file (.pic) and the Photoshop file (.psd). If there's space, include the Master Flat, Master Dark, and Cosmetic file as well.
Omega Centauri Globular Cluster Image © John Drummond. Used with permission.
© 2001-2005 Simon Szykman and Teri Smoot
