The Horsehead Project:
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As wonderful as the Duncan image appears on a superb photographic print, it should be noted that even as late as 1920, the original plates were more sensitive to the 'greenish' hydrogen-beta wavelength (486.1 nm) radiated by IC-434 than its ruddy hydrogen-alpha light (656.3 nm) as modern films or digital sensors would be: thus we now perceive the 'plume' of light that appears to extend far above the 'top' of the Horsehead, which modern amateur pictures often register out as far as approximately 40 arcminutes or further (and which Dr. David Malin's
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Even the ancients noticed the discontinuity in the Milky Way in their dark, pre-electric light, skies. As soon as efficient optics could be trained on the heavens, early telescope observers -- especially comet hunters -- noticed dark spots, lanes, rifts, and regions of fewer stars. William Herschel suggested that some of the blankest and blackest of these might be "holes in the heavens"; a more likely understanding came about through the sky study by his successors, who (as we see in these articles) finally determined by means of photography and spectroscopy that there were dark obscuring bodies in the deep sky (though the ones that were most clearly noticeable are in our own galaxy and therefore, on a cosmological scale, are very close-by: this is now termed "dark interstellar matter". The Horsehead consists of such material.) But the "dark matter" now of concern to cosmologists who are trying to determine answers about the formation and structure of the universe; its future expansion (or contraction); and the causes of the remarkable large-scale clustering of galaxies, is being indirectly inferred from observations on a scale much larger than that of our own local environment, the Milky Way galaxy. This link shows a "Dark Matter Map" by Drs. Kneib, Ellis, and Treu illustrating the distribution of the invisible matter on the scale of an enormous galaxy cluster approximately 4.5 billion light-years away: the dark matter's distribution appears uneven, but congruent with clumps of the matter that is luminous and therefore directly observable by the instruments of the Hubble Space Telescope. (Sadly, the fascinating and lucid introductory tutorial about Dark Matter, Cosmology, and Large-Scale Structure of the Universe, prepared by Jonathan Dursi, a physics graduate student of Queen's University in the UK, seems to have been taken off the Net recently.) To conceptualize the difference between the two -- dark interstellar matter and dark matter -- visit the web article by Martin White, Professor of Physics & Astronomy, UC Berkeley. |
Observing the Horsehead With Small InstrumentsMany of the professional astronomers I have spoken to are surprised that small telescopes can reveal the Horsehead. I was delighted to discover that Dr. Dale Cruikshank, Research Scientist at NASA Ames in Mountain View, had not only seen the object himself at age eighteen, but also became interested in astronomy because of the same event that stimulated me, the 1954 solar eclipse that was visible over large parts of the midwest.Dr. Cruikshank saw the Horsehead as a student at Iowa State, observing with an 8-inch short focal-length Newtonian at low power in the then pitch black skies at Ames, offering a naked-eye limiting magnitude of about 6.5. As a planetary astronomer, a pursuit which later became his career specialty, he had not spent much time looking at deep-sky objects, so observing the Horsehead was a satisfying accomplishment. Dr. Cruikshank considers himself not only a professional astronomer, but also an avid amateur observer of the heavens, and has commented that many professionals may have lost touch with the sky's beauties. Recently [written in 1990] he pointed his backyard 8-inch telescope at the Zeta Orionis region, but was sad to note that Santa Clara valley stray photons had wiped out any trace of the Horsehead.
Shiloh Unruh and I looked in vain for the nebula in early spring of 1987, hampered by the necessity of using too high a power and viewing across the lights of the Santa Clara Valley. We saw the mottled, fluffy background of IC-434, but no definite Horsehead, even employing a light-pollution filter. Retired Lick staff astrononomer Eugene Harland similarly reported his inability to see the Horsehead in the 36-inch Crossley's guiding eyepiece, possibly because of the light loss of numerous uncoated optical surfaces. I was skeptical of reports by various amateur astronomers claiming to have seen the Horsehead in instruments as small as 3 inches of aperture, so I set up an observing program to test the smallest possible instruments. We found that on a good, dark night the Horsehead is relatively easy in a huge 17-inch "light bucket" Dobsonian, especially if a Lumicon "Hydrogen-Beta" [TM] filter was employed, but what of the smallest apertures? Between October 1989 and the end of March, 1990, about thirty viewing sessions were at least in part devoted to the Horsehead. The sites chosen were the mountain range opposite Mt. Hamilton's Lick Observatory, private property in the Santa Cruz mountains south of San Jose, at about 3400 feet altitude, and the famed Fremont Peak, nearly 3,000 feet above the beautiful Monterey Bay and Salinas Valley. Four observers (author Waldee, Richard Page, and the father-and son astrophotographers Ron and Ryan Wood) were present at many of the sessions, to verify results so that just one subjective impression was not the final arbiter of whether or not the faint Horsehead was sighted. In some cases, by necessity, I observed alone, and my claims must be considered as unverified. Proper Dark Adaptation
Stray light will probably diminish any chance of seeing the faint differences of dark contrast that are necessary to be able to detect the Horsehead. You probably can't just "look into the eyepiece", for light all around the periphery of vision -- even in the darkest skies -- will spoil the view. It is necessary to have, essentially, total blackness outside the immediate eyepiece ocular field, which you may be able to obtain by extensive shielding by one manner or another. Having at first tried to achieve this by placing a black hood over my head, I was constantly troubled by difficulties in breathing, continually fogging the telescope or binocular eyepieces. Finally, I settled on using Rose Star Products' red ASTROGOGGLES [TM] during the long period of time that it took to set up my telescope, and during all times that I was not actually observing. I also made certain NOT to try to find the Horsehead right away at the start of a viewing session, but only after I had become fully comfortable and dark-adapted.
Perceptual experiments have proven that the human eye can predictably respond to as little as 50 to 100 photons, and the classic Hecht study of 1942 (which I first consulted in 1989 at the UC/Santa Cruz science library, though now it is handily on the web) determined that absolute retinal sensitivity per rod was about 5 to 14 photons of blue-green light (the photon loss of the entire eye mechanism, from cornea to retina, is at least 90%): but with care, many more photons than necessary will fall on the eye, conditions having been optimized. Furthermore, the greenish hued hydrogen-beta nebular line (486.1 nm) radiated by the nebula IC-434 is close to the wavelength (510 nm) at which the retina -- when dark-adapted employing "scotopic" vision -- is most sensitive (although the stronger red wavelength hydrogen-alpha radiation given off by the nebula is still much too weak, as collected by small telescopes, to be perceived by scotopic vision.) Thus it is not essential to use very large telescopes to see the Horsehead. Efficient Viewing TechniquesIt is crucially important to obtain best contrast; even a huge fast Dobsonian telescope would not, on some occasions, delineate the faint distinctions of light and dark that would be shown in a small well-baffled refractor. The wider the visual field, the more light scatter may be encountered from the bright stars of the region, especially with a large-aperture scope. IC-434 is fairly bright, and was repeatedly observed in binoculars as small as 42mm of aperture. But what is necessary to discern the small, dark "notch" of the Horsehead, against the glow of the nebula, is the means to avoid stray light scattering in both the atmosphere and the instrument (whose surfaces should be free of dust.) As far as sky conditions are concerned, we weren't usually able to spot the Horsehead with small apertures (even in the relatively clear and steady California vistas) unless the immediate celestial region was no farther than within 30 to 40 degrees of the zenith. If the object was lower, the blurry, glowing, and polluted air near the horizon wiped out the faint details of contrast, obscuring the Horsehead even though IC-434 might still be visible.
You may view or download a version of our 52 Orionis finder chart -- prepared from the open-source "freeware" program Cartes du Ciel -- in monochrome, with black stars on white for printing, by clicking this link. If one could also clearly perceive the faint peppering of stars in the background of 52 Orionis, the sky might be dark enough to get a glimpse of the Horsehead in binoculars, even if there was some light pollution at the horizon. I tried what is perhaps the ultimate test of seeing the object with small apertures: first, I sat with a dark cloth over my head for at least 45 minutes (following the process devised by William Herschel) until I was sure that my eyes were profoundly dark-adapted. Then a viewing assistant carefully inserted a pair of 1.25" diameter Lumicon H-beta filters into the rubber eye-guards of a Carton "Adlerblick" fully-multicoated 8x42 binocular, and handed it to me when I was ready to give it a try. (Even the 8-power binoculars shook so much while hand-held that I had to steady them with a prop. But this also helped me to be able to sweep them back and forth slowly without losing my field: sometimes this technique will assist in detecting faint details.)
Well, none of the other three observers present during the viewing session tried the feat, not being willing to sit still under the dark cloth. Only the author -- by now, an admitted "Horsehead fanatic" -- was consumed by the determination to try it, no matter what the cost of boredom and inconvenience. So, you are willing to regard this claim as "not proved", since we can't corroborate it by even one other test subject under the same set of circumstances. At least we can say that the claim is made with sincerity, and since I had seen the Horsehead innumerable times in telescope eyepiece views, even at powers of magnification as low as 16x, I was trying NOT to imagine it but to be objective! Repeated confirmations proved that the Horsehead could indeed be discerned in relatively small instruments with diligence, excellent dark adaptation, and the use of the proper filters to reduce both man-made light pollution and natural sky-glow. In 1825, the Czech physiologist Jan Purkinje published his experiments with his own visual perception of colors in varying degrees of illumination, finding that night vision is generally restricted to the blue and green hues. Thus, despite the glorious red color in long-exposure photographs of IC-434's hydrogen-alpha radiation, the human eye will respond only to the cloud's weaker hydrogen-beta line at 486.1 nanometers. Such is the encroachment of man-made light pollution that in virtually no instance could we hold the Horsehead's image with direct vision without employing the Lumicon h-Beta Horsehead filter[TM], with about 95-96% transmission of this narrow band of blue-green radiation, a small percentage of transmission of Oxygen-III lines, and an almost total cutoff at other frequencies. Our forefathers had a much better chance of studying the Horsehead in their pristine, dark skies (though as our research has shown, even after the object was known, it was rarely seen: optical design and fabrication was too primitive at the turn of the 20th century, compared to the products available today even to amateurs.) The Lumicon filter is designed for a telescopic exit pupil of 4 to 7mm. (A quick way to calculate its appropriate use in a given telescope is by employing the Eyepiece software program that I co-wrote with Ron Wood.) Since the typical dark-adapted pupil relaxes to an opening of 5 to 7mm, we must put all of the light gathered by a small telescope onto the eye's retina, or we have virtually no chance of spotting the Horsehead. Exit pupil is determined by dividing the light-gathering aperture of the optical instrument (in millimeters) by the diameters of magnification employed; thus, a pair of 7 X 50 binoculars produces an efficient exit pupil of 7.14mm (50 divided by 7), and all of the light leaving the ocular enters the eye. Though important for photographic speed, a fast telescopic focal-ratio is not directly essential for seeing dim objects; focal ratio will determine the range of magnifications provided by one's set of eyepieces. Therefore, the short focal-ratio "richest field" design may often be ideal for obtaining the low-power and large exit pupil needed.
A major difficulty in using an efficient wide field, low power 7mm exit pupil to observe the Horsehead is the strong blue light scattered in the optics from the nearby belt star Alnitak (Zeta Orionis), a triple star system shining at nearly 1.8 magnitude. We found that with the more powerful instruments above 6-inches aperture, it was necessary to employ a smaller exit pupil than 7mm to avoid ruining one's dark adaptation due to Zeta's brilliance.
Your Perception, and Oxygen LevelsA mystique about using oxygen has crept into the lore of observers, but since the body's breathing mechanism is regulated by the carbon dioxide in the bloodstream, and hyperventilation can bring on breathing difficulties, we wanted to be extremely cautious about experimentation. Through our old Lick Observatory friend, Ron Laub -- now the site manager at the Keck Telescope in Hawaii -- we got into contact with Dr. Kelvin DeGinder, M. D., an FAA medical examiner and the high-altitude consultant to CIT/Keck, who explained that oxygen bottles are a necessity for the workers and astronomers at the 13,798 foot summit of Mauna Kea, but merely for adaptation to normal sea-level bloodstream oxygen saturation. Night vision, DeGinder verified, does indeed degrade about 25% at that high altitude due to hypoxemia, when the typical oxygen blood level drops to about 80%. Breathing extra oxygen will raise the arterial oxygen level to the normal sea level amount of 98 to 99% saturation. However, prolonged breathing of pure oxygen cannot raise the saturation level to much more than 100 to 101% of normal, and will have no appreciable effect in enhancing night vision beyond sea level capacity. If the altitude is sufficiently high above sea level (as Dr. DeGinder and friends experienced themselves in 1985 when viewing Halley's comet at Mauna Kea), using extra oxygen may increase contrast, to quote one of the observers during the occasion, ''like turning up the brightness on a TV picture." But one does not obtain super-vision beyond average human capacity: the proper oxygen level merely restores normal perception. Our sea-level atmosphere contains only about 20% oxygen, and a higher concentration can produce the effect of extreme hyperventilation. I was not at all anxious to get dizzy or even to pass out on top of a 3400 foot mountain top from an oxygen overdose! Nevertheless, I did try taking a few breaths of oxygen mixed with normal air while viewing through the telescope eyepiece. As I stared at the field, the sky background actually became a bit lighter than normal, and seemed to see more visual "noise" in the image, and less contrast. I repeated the procedure while observing the Pleiades without optical aid, and found that I experienced a slight impairment in concentration for a few moments. No improvement here, at least at 3,400 feet above sea level. It is undoubtedly better -- and safer -- to enhance one's natural dark adaptation in the long run by maintaining a healthy diet with sufficient vitamin A, preserving the eye's ability to form rhodopsin (visual purple). A low vitamin A diet for two months or more can cause night vision to drop to less than 20% of normal. But an overdose of vitamin A can be toxic or even lethal, so play it safe by simply eating plenty of carrots and fresh spinach! AND NO SMOKING!! The Weather and Sky Conditions: Critical
The results of our test are not to be construed as a critique of the individual telescopes used, which may perform with much greater efficiency under better circumstances. Winter viewing in Northern California is very rough and unpredictable; would that the Horsehead was high in a summer constellation, or that we were closer to the equator. Or, how about emigrating "down under", where Orion is a balmy summer spectacle? We tried for as much consistency as possible, setting up as many as six telescopes close together at the same site, though the table shown further below displays results obtained over a period of about six weeks' time, ending in late January of 1990. In essence, at least one of us was able to see the Horsehead with all of these instruments under some conditions of magnification and filtration. Even Rudimentary Equipment May Work
And you have a small aperture -- but suitable high quality -- telescope, be sure to try it out on the Horsehead: you may be pleasantly surprised! At the very least, the search to confirm a view of the object will help hone and refine your observing techniques. Locating the Correct Field of ViewNaturally, a good finder chart will be useful, if you aren't used to looking at the Horsehead through your particular telescope and eyepieces. Here's one we prepared from the open-source "freeware" program Cartes du Ciel, with our own added labels. You may view or download a finder chart in monochrome, with black stars on white for printing, by clicking this link.
An amateur astronomer friend of mine once came up with an analogy of visualizing an arc of three bright objects near the Horsehead as being arranged like the "dish of a radio telescope, whose focal point is the Horsehead nebula." I have taken a photograph by my friend Ryan Wood and added labels to some of the bright stars plus NGC 2023, in the immediate region, showing the arc of stars that seem to point right at the "snout" of the Horse. You may right-click and download or print out this chart for use at the eyepiece. 
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The difficulty of obtaining a dark enough sky had discouraged my own searches for many years. Finally I had an opportunity to use a telescope that one assumed would offer sure-fire success, a 22-inch Cassegrain at Lick Observatory, the Tauchmann telescope, utilized for non-research purposes (shown in the author's photo: the large dome on the left is the great 36-inch refractor; the small dome on the right is the building that housed Barnard's old telescope, the Crocker, and now the Tauchmann instrument, in service since the mid-1950s.)
Of prime importance in achieving success in detecting faint nebulae: observe in the best possible night sky, showing faintest stars and least amount of background glow, only after letting your eyes attain full dark- adaptation (in the snapshot, which I took in 1989, Rich Page sets up his refractor while the fog gathers at lower elevations, so that both he and the telescope are ready to go later when the Horsehead is highest in elevation, and the light pollution is decreased during the wee hours of early morning.)
As has been published in the February 1990 ASTRONOMY magazine article "The art of Seeing" by Michael Porcellino, dark adaptation requires resting the eyes for at least 30 (and preferably 90+) minutes, essential for the ability to observe the dimmest sky objects. Dark nebulae require the most profound dark-adaptation, and fully-dilated pupils to admit as much light as possible (youngsters' eyes may open up to 7mm, while middle-aged persons, such as this observer, are lucky to have a maximum 'entrance pupil' of 5mm after their eyes get used to very dim light levels.) 
When looking through the eyepiece, I also employed a home-made light shield created from a welder's face mask, which I modified by taking out the original glass filter, substituting a black plastic plate containing a hole slightly larger than the outer diameter of my eyepiece; of course, care must be taken to avoid touching the ocular and shaking the scope. But the shield helped mask the background light so that I could concentrate on the exceedingly fine distinctions of contrast in small instruments' images of the Horsehead. This homemade device is also very useful for non-magnified observations of the sky by means of a 2" diameter nebular filter, as all ambient light outside of the filter's central periphery will be blocked, and the filter is held at approximately the correct angle. (The image of Regina Roper wearing the goggles was derived, unfortunately, from a black-and-white photocopy of the original snapshot, which I've tried to colorize to highlight the red plastic filter.)
We also found it essential for the Orion belt region to be situated in a part of the sky that reached at least a naked-eye stellar limit of about 6th magnitude. The star 52 Orionis, of 5.27 visual magnitude, is situated two degrees west of Betelgeuse inside the boundaries of the constellation, and proved to be a good indicator of darkness and transparency. It was absolutely crucial to be able to hold the image of this star with direct vision in order to be able to see the Horsehead in small aperture optics -- but always requiring also the aid of a Lumicon "Hydrogen-Beta" [TM] filter or comparable make -- which may provide more than an entire magnitude of contrast improvement. [At time of this article, only Lumicon Company supplied one; now there are several fabricators.] 
Yes: indeed, the small aperture worked! In my attempt to draw something of what I had perceived (at left): IC-434 was quite obvious as a long stream of light, below Alnitak, the left of the two bright stars in Orion's belt shown in the sketch; and just at the correct spot, a smallish dark speck, the Horsehead, was detected! Of course the image scale at 8 diameters of magnification is tiny, so one has to have proper expectations of dimension well in mind, in order to know what to look for. Is this a valid claim? 
And the best views during our test were achieved with high-quality refractor telescopes, with higher contrast than reflectors employing secondary mirrors that block some area of the full aperture and diminish the blackness of the dark sky background. The view through Rich Page's 7-inch aperture Astro-Physics "Starfire"[TM] apochromatic refractor was excellent, as I attempted to show in my sketch: here, a 20mm Televue Nagler [TM] eyepiece and Lumicon h-Beta filter [TM] work together, at 80x, to provide a gorgeous wide field, with no trace of light-scatter from Alnitak: nebulae 
Dr. Jack Marling of Lumicon suggested using a good quality narrow-field Kellner eyepiece to help restrict some of the light. I unscrewed a 40mm Kellner and easily inserted a black-paper occulting mask to block half the field of view, further eliminating the effects of Zeta and making it easier to discern the barely-visible dark spot; one has only to rotate the eyepiece to block Zeta and Sigma Orionis for the Horsehead to pop into view. Unfortunately, decent quality coated Kellners are much harder to find now, in 2005, than they were in 1989-90 when our research was conducted for the original article: a Kellner is useful because the field lens is in the focal plane, and the occulting mask is therefore in sharp focus. Astro-marketing now tends to diminish the "old fashioned" eyepiece designs, but you may find that some oculars with apparent fields of 40-50 degrees are, for all practical purposes, very similar. It may be possible to locate a "sweet spot" where the occulting mask is effective, between the field lens and the barrel opening. 
For instance, I happened to acquire a number of identical Orion "Sirius" 25 mm Plössl oculars with several telescopes of that brand. I determined in an easily accomplished experiment that I could place a black plastic or heavy cardboard occulting mask inside the metal barrel (or that metal wire crosshairs could be added): the ocular yields relatively low power but the mask permits excluding bright stars while looking for close-by faint nebular details.
Using the widest-field, multi-element oculars may not always be the best choice for viewing the Horsehead if you employ a reflecting telescope that exhibits light-scatter. It is desirable to get NGC-2023 out of the field: its bright whitish glow will distract the eye and rob the image of contrast. Needless to say, it's almost impossible with a large aperture to allow Zeta or Sigma Orionis to be even at the far periphery of the field of view. Basing our graphic on the eyepiece simulation picture of the Horsehead in our old DOS astronomy program "Eyepiece", we offer a simulated ideal view with a large aperture, efficient scope, such as the 17.5" f/4.5 Dobsonian that the author used many times to see the object in dark skies, employing a Lumicon h-Beta filter. Here, the field of view is about 20 to 25 arcminutes, with a magnifying power of about 100 diameters, which could be achieved with a narrow-field 20mm ocular. The exit pupil will just be in the lower range for best contrast with the filter, and the Horsehead will tend to seem a bit blacker than the surrounding sky, the top edge being outlined by faint glow from IC-434.
The challenge in observing the Horsehead with small aperture optics was not just to see it at all, but to find out how high a power, and how narrow an exit pupil, could be employed, as a test of optical quality and contrast. But any comparison made under normal sky conditions is by definition unscientific and anecdotal. From hour to hour, especially in winter skies, the transparency changes continually. Isolated regions of a generally bad sky can be much clearer and steadier than others, as I found when clouds parted just around the constellation Orion on one of the worst winter nights, and the Horsehead briefly but strongly stood out in the wide field of view of my "RFT" telescope,
a 4-inch aperture "Astroscan"[TM] (which I attached to an equatorial mount with motor drive to keep it centered on the field for the convenience of all four observers in the tests.) Yet even though the Horsehead could be seen during one test in "iffy" weather conditions with partial cloud cover, I could barely observe obscured Jupiter in another region of the sky. Important lesson learned: if at all possible, employ a telescope with clock drive, and be patient!
Just for fun, I put together a "5 minute telescope'' from a used 60mm photocopier lens mounted in a few plastic plumbing parts, and this proved to be one on the best instruments for viewing the beautiful dusting of nebulosity in IC-434; the Horsehead was indeed discernible when smoothly sweeping the instrument back and forth across the field! But only in the 7-inch and 8-inch scopes did we have the satisfying feeling that we really saw something, the kind of palpable presence Edward Barnard was seeking when he studied dark nebulae with the 40-inch Yerkes refractor. So, you owners of 10-inch or larger reflectors: do out and grab the Horsehead before everybody on your block has claimed it.
Of nearly the same quality is a remarkable color picture made by one of our intrepid four: the son of Ron Wood, then a precocious youngster of 14! Ryan Wood created this image with one exposure, below, on 35-mm Kodak Ektapress 1600 film with his camera attached to the prime focus of a home-made f/5.2 Newtonian telescope, using a forty-minute exposure (employing also a general-purpose broadband light pollution-reduction nebular filter -- a Lumicon "Deep Sky" [TM] model -- which has slightly altered the color palette and shades of red, but prevented premature sky-fogging of the negative.)
