Here are some of the functions of "Eyepiece", and how you may use the program...

Setting Up Your Simulation of Telescope and Eyepiece.

First, you must define your telescope. The program comes with one pre-defined scope system; you may load this or start from scratch, specifying the telescope aperture (inches/millimeters), and either the focal length (in/mm), or focal ratio (f-ratio, f/...). You may also use the typical telescope value database instead, which contains information about 100 conventional instruments ranging from 50 millimeters to 25 inches aperture (just about everything imaginable that you are likely to buy, build, or use):

You may start your analysis by specifying a particular eyepiece, giving the program your own known set of specs -- focal length (mm) and apparent field (degrees) if you are already an experienced scope user -- or by choosing one from the program's Eyepiece Value Database...

...Or, if you prefer, by selecting one of a number of scope/eyepiece performance goals as shown in the menu below.

The program's principal author still finds this menu offers INVALUABLE assistance! Just the other day we were building a small finderscope, made from a good 50mm achromatic lens of known focal length. In only moments, "Eyepiece" helped us make up a list of the oculars we might want to use in it, based on our requirements for FOV and exit pupil: much faster than running numbers on a pocket calculator from various eyepiece formulae!

If you are a novice, you may need assistance from the program's Help module, which includes (at this point) a text description of the common eyepiece design types (orthoscopic, Plössl, wide-field, etc.), as well as a visual chart showing the typical eyepiece element layout.

Let us suppose that you chose a 20 mm focal length ocular of a specific type: a typical Plössl. "Eyepiece" now does all the appropriate calculations using your previously-defined sky darkness factor (naked-eye stellar limiting magnitude) and then runs the numbers for ideal operating values.

Important values given above include magnification; the estimated visual field of view (sometimes referred to as "true field" if the eyepiece field is actually measured accurately by eye, with star drifts); the exit pupil (a significant value that is an indicator of potential brightness of certain types of objects); scope diffraction limit ("Dawes Limit", assuming perfect seeing and flawless, well-adjusted optics); the recommended magnification for observing closest double star separations (for doubles of moderate and similar brightness); recommended options for use of LPR/nebular filters if within range of the recommended exit pupil value; estimated stellar magnitude limit; and some general suggestions of the appropriate type of use for celestial viewing as well as the category of the optical system among the typical scope designs (richest-field; narrow field planetary/lunar scopes; general purpose deep sky scopes ranging from "fast" to "slow" focal ratios.)

In the example above, the user chose the program's default "naked eye stellar limiting magnitude" of 6.5 magnitude: a "dark sky". So, it shifted to the "rural sky" series of exit pupil values appropriate for the nebular filters for a sky with little or no artificial light pollution. If you had defined a brighter sky, "Eyepiece" would choose instead the "city sky" values, taken from parameters developed by photophysicist Dr. Jack B. Marling (and used here with his permission.)

A particulary useful parameter is the calculated value of Widest Estimated Field of View for a defined focuser barrel size. Two-inch oculars may yield, all else being equal, a wider field of view than comparable 1.25" barrel eyepieces. Suppose you DO have a 2" focuser: is it worth the money to move up to more expensive 2" barrel eyepieces? The increased FOV value may be worth it, for specific wide-angle views of some of the sky's most memorable treats: the Double Cluster, the North American Nebula, sweeping through the Milky Way, etc.

Selecting Objects to View: Double Stars, or Deep-Sky Objects.

For double star enthusiasts we mated an older program, "DoubleStars 2.0", originally developed as a module of our earlier "Redscope" application (sorry...it is so old that we never added mouse support to this section!) Pressing "S" for "Stars" on the main system screen menu starts this program, as shown in the splash screen below.

After an introductory explanation -- and the option for the user to define the telescope's image orientation -- for example, are you using a star diagonal in an SCT, or a typical Newtonian? -- you are able to choose from database lists of approximately 600 selected doubles in the constellations visible in the northern hemisphere (items chosen by John Sanford in his book "Observing the Constellations", and with data provided for our inclusion by permission of the UC/Lick Observatory.) Let us show part of a typical list, below:

The program next allows the user to decide, based on the separation factor of the selected double, one of four choices of suggested magnification (for close, similar-magnitude doubles; doubles of widely-varying magnitude differences, etc.); then according to the choice, an exact theoretical eyepiece focal length is "installed" in the telescope currently defined. You will note that the position angle of the double is re-calculated, below, since the scope image orientation alters the actual measured sky P. A.: a useful feature if your diagonal mirror inverts or reverses the image.

If you chose from the menu options -- shown immediately above -- to search the database for the recommended magnification using conventional oculars, the program searches for the closest value of eyepiece in its internal database. In this case, below, it installed a 2X Barlow to achieve a value closest to the exact magnification goal (in this case, 203X, slightly more than the target value, as the program always opts to be on the higher side of the target if the EXACT value is not available. You needn't worry if you can't duplicate this precise target value; experiment to see what you can achieve with the equipment at hand. Double star resolution by eye will vary enormously depending on sky conditions and the acuity and experience of the observer.)

Needless to say, "Eyepiece" attempts to wade out 'impossible' doubles that can't be split if (say) the diffraction limit of the system is inappropriate; but it cannot predict bad seeing or misaligned optics!

Note: On most modern Windows systems, even with XP, we can make the Double Star program work correctly when "Eyepiece" is used from Windows, rather than booting the system in DOS. But occasionally, "Double Stars 2.0" won't start from the Eyepiece menu. If you have this problem, the trouble is likely the Windows memory settings for DOS programs. This seems more prevalent in XP (Home or Pro) than in Win 98SE or ME, which seem to provide more default setting of memory for DOS programs. If all else fails, you may find an alternative "workaround" described in our readme file under the section "(b) DOUBLE STARS MEMORY LIMITATIONS."

Perhaps most observers spend their time with typical deep-sky objects -- star clusters, globulars, nebulae, and galaxies -- and so we did not slight this important type of observing! "Eyepiece" contains databases of hundreds of appropriate objects, again from Sanford's book, sorted into categories of specific types. Below, for example, is the first page of one of the nebulae databases, with the famous "Lagoon" nebula (Messier 8) on the top line, ready to be selected for calculation.

Now, if we can be so bold as to claim it, some MAGIC happens! "Eyepiece" does a very thorough analysis of the object, based on its characteristics (including angular diameter, most easily- visually- detectable nebular line, visual magnitude, and estimated surface brightness): these values are integrated into the optical system of the defined telescope under the sky darkness conditions that have been defined earlier. The preliminary calculations are shown, below, before the main program completes the process of selecting optical values for potential optimal viewing.

Here's what we wrote in the appropriate Help file for the Observer section of the program, in explaining the general principles of our "Object Visibility Calculator":

  
=== (G) EYEPIECE OBJECT AND TELESCOPE ANALYSIS PROCEDURES =============

      This program performs an extensive analysis of the selected
  deep-sky object.  Diffuse nebulae, planetaries, and galaxies are dealt
  with by category and subset of object to best predict the magnification
  range (and optional Lumicon filter) that will be effective for
  viewing them in the defined sky and telescope.  We are grateful to
  Dr. Jack P. Marling of Lumicon for his advice and assistance in
  many details of developing the analysis algorithms.

      Once we achieved mathematical functions capable of any quality
  of analysis of the size, brightness, and overall prediction of the
  visibility of an object through a given telescope, it was necessary to
  compare the accuracy of the prediction against real-world experiences.
  Then many adjustments to parameters (fudge-factors, if you like) were
  performed to tune the various formulae and procedures.  We hope that
  the analytical results will be very helpful as guidelines, if not
  offering absolutely deadly accuracy!

      In general, the analysis of extended objects is performed in
  the following order:

    (1) Data are read in from the datafile of the object selected.
    The visual magnitude, object type (including some "hidden" object
    character description values), and size of angular diameters are
    assigned to program variables.

    (2) A calculation of the ROUGH estimate of object surface brightness
    is performed, using the area of the object, compensating somewhat
    for its type and character, and the best estimate of visual magnitude.
    This value will not be as accurate as professional results obtained
    by using standardized data, but will be better correlated to the
    general brightness characteristic of the object than the old,
    classic "visual magnitude" figure alone.

    (3) Compensations are made for the amount of light pollution present
    in the defined sky, and the ability of a given telescopic light-
    gathering aperture to detect the faint outer reaches of an extended
    galaxy or nebula.

    (4) Further adjustments are made to correct for extremely large and
    faint objects that cannot fit into the field of view of a given
    telescope, or for the nature of dark nebulae that must be viewed
    against a brighter background.

    (5) Based on the character of the object as either a broadband-emitter
    like a galaxy, or a narrow-band emitter like a hydrogen or oxygen
    nebula, a field of view or exit pupil are selected.  For galaxies,
    the goal is to place the object in a proper field where it will be
    clearly visible against the sky background.  For nebulae, the general
    criterion is the correct exit pupil for efficient detection by the
    eye, especially as aided by any appropriate Lumicon nebular filters.
    If the object is stellar, its visual magnitude is compared with
    the telescope's stellar magnitude limit without filters in the
    defined condition of sky darkness.

    (6) The eyepiece database is searched of all appropriate oculars in
    the defined barrel size; the object diameter is measured against the
    available field of view of the telescope.  If the focuser barrel is
    set to less than 2.0 inches and the object will not fit fully into
    the widest available field of view, the user is notified of the
    possible option of trying again after resetting the telescope
    definition to use wider-field larger-barrel oculars if physically
    possible.  The eyepiece (and appropriate filter, if any) that are
    closest to the predictions of the analysis are "installed" in
    the telescope, and all calculations are made of theoretical optical
    performance.

    (7) A "Visibility Prediction" value is determined for the sky,
    telescope, and object.  The result is characterized as:

       (a) BAD! : The object probably cannot be seen under the
       conditions defined with this telescope;

       (b) POOR : The object MAY be visible if great effort and skill
       in observing are utilized;

       (c) MARGINAL : The object probably can be seen but may not
       be particularly satisfying aesthetically, and can be viewed
       much better under more favorable circumstances;

       (d) GOOD : There will be no special difficulty in detecting
       the object and its characteristics;

       (e) VERY GOOD : The object should look fine, and will be
       suitable for satisfying viewing appreciation if within the
       telescope/eyepiece field of view.

    The most subjective aspect of our procedures, the visibility
    prediction is weighted by many factors: sky brightness from
    light pollution; faintest stars visible; size and nature of
    object; and performance of telescope.

    To see how it can work most reliably, use ONE telescope and set the
    naked eye stellar magnitude limit in several different steps from,
    say, 3.8 to 6.5 magnitude, using the '[c] Configuration' option on
    the main SYSTEM SCREEN menu.

    The visibility prediction for a typical diffuse nebula like M20 or
    M8 may thus change from 'POOR' or even 'BAD!' to 'GOOD' or 'VERY GOOD'.
    Not quite so much change may be evident when stellar objects are
    chosen, as long as their visual magnitudes are within grasp of the
    light-gathering aperture.

As far as we know, this unique process -- developed by Steve Waldee with some suggestions by Jack Marling -- is the ONLY one of its kind in any amateur astronomical observing software! As shown below, the program indicates a suggested "starting" value of eyepiece (and possibly filter, if appropriate) in case you are not a seasoned observer. Even if you are, you may enjoy playing with the program by testing different scope and sky conditions, to see how YOUR opinions jibe with ours! (Or don't jibe, as the case may be...)

Below, the program has completed the process of selecting a practical suggestion of eyepiece and filter. You need not be afraid to deviate from our suggestions, especially if limited by your selection of equipment. Though the calculation process is mathematically rigorous, OBSERVING is not; in fact, good deep-sky observing is often more of an art than an exact science!

And some deviations from printed values of sky catalog parameters for some of these objects are to be expected. For instance, our calculation of estimated surface brightness of certain extended objects, or the effective diameters of galaxies, are based on VISUAL perception with human eyes with respect to telescopic efficiency, not instrumental values acquired by spectroscopy or micrometrics.

Finally, if you'd like a little amusement, try to ask for an analysis of objects that you know are almost impossible to see in a given scope. For example: define a little 60mm aperture, 700mm focal length refractor with 0.965 inch barrel eyepieces; tell the program that you are observing in a bright city sky. Then ask for the Horsehead Nebula! "Eyepiece" knows that THIS particular observation is impossible, and doesn't mince words. It will know, too, that many very large diameter faint nebulae will be invisible, as well as any number of extremely faint NGC and IC catalog objects. (It does not know because the "answer" has been encoded somewhere; it CALCULATES this!)

How Does The Object Look to the Eye, or Appear in a Photo?

The telescope-aided human eye will usually show no color in galaxies, and only traces of pale shades in some bright nebulae (as explained extensively in the documentation and Help sections of our program.) Beginners must expect not to see the glorious shades of deep-exposure film or CCD images. A few of the famous sky objects are shown by "Eyepiece" in scanned and processed images that attempt to replicate eye views with particular types and apertures of scope, in light-polluted and dark skies; and a few astrophotos are included. For instance, in the panel below, M-8 is seen at left in a simulated view under skies near a city; then in a dark sky; and finally on the right in a color photograph. Expect to be able to experience something like these views with, say, an 8-inch aperture scope.

"Eyepiece" has a menu of objects for examination, including examples of planets, solar images, galaxies, clusters, nebulae, and one spectacular comet. Each item includes a brief description as well as an explanation of the telescope used for the simulations and final photos. A slideshow function is also available, to show the images either in order or by random selection.

Unfortunately, what looked (barely) OK on a 13-inch 1989 8-bit VGA monitor now looks pretty poor on today's high-resolution gigantic 16-million color CRT's! So, the graphical functions of "Eyepiece" are now obviously obsolete and rudimentary; but you may be amused that we at least tried to produce representative simulated views to dispel improper expectations of beginning observers (who are almost always dismayed and unimpressed by their first telescopic views of deep-sky objects.) And, if you cringe at the pixellated object pictures on your modern high-fidelity PC screen, you might try viewing them from the far side of a large room!

Other Visual Comparisons of Objects in Diverse Telescopic Views, on the Web

Here are some links to various attempts by both amateurs and commercial telescope makers to show objects in different telescope apertures:

• Astronomy Sketches with a 4.1" Traveler Telescope by famous Sky & Telescope magazine "Deep Sky Wonders" columnist Sue French;
• Deep Sky Sketches by Jaakko Saloranta;
• Visual Deep Sky Observing by Andreas Domenico, Breitenbuch, Unterfranken (Germany)
• Comparative Messier Digital Object Sketches With 11x80, 20x100 and a Tasco 60mm, by Ioannis Galidakis;
• Comparison of M-13 in Various Apertures, on the website of Obsession Telescopes, makers of magnificent large-aperture Dobsonian state- of- the- art scopes;
• Aperture vs. Light Pollution, with comparisons of M-31, M-38, and M-42, on the website of Tony Flanders, writer for Sky and Telescope;
•  Visual Astronomy: Telescope Aperture and Detecting Detail in Astronomical Objects, An Example Using The Whirlpool Galaxy, M51, on the fascinating website of Dr. Roger N. Clark;
• Drawing the Deepsky: on this elegant site of Fred Hissink, click on the "Gallery" links for drawings made under different magnifications.

Our gratitude is extended to "canopus56", a contributor to the newsgroup sci.astro.amateur, for posting several of the links above on May 20, 2006.

Using "Eyepiece" to calculate prime focal film exposure times

Anybody still using film? (We don't; CCD's are the thing today!) But, back when "Eyepiece" was developed, the webcam or home computer CCD imager was non-existent. Astrophotographers at the beginning level were likely to be using 800 to 3200 ASA rated color film, and the real experts used gas-hypered Kodak Tech-Pan 2415 b&w emulsion. Ron Wood, our partner in developing the software, used Kodak Ektrapress 1600 multispeed color print film, gas-hypered, for the photos he contributed to the program: this is professional print film that was hard to find in the '90's, and is discontinued today. Dr. Jack Marling was fond of using hypered slow color film for planetary nebulae with extremely high surface brightness, for the resulting large signal-to-noise ratio and rich hues. Everybody who did extensive color film astrophotography in the bygone days had his or her preferences and prejudices...

So, we included a function to calculate prime focal exposure times for the deep-sky objects in the "Eyepiece" program's databases, optimized for four film types/speeds that were commonplace around 1992.

As above in the discussion of the visual observation of Messier 8, we show the program's calculations of approximate exposure times and bracketing range for varying sky conditions. If the optics are appropriate and the object brightness is within range, the program may also recommend using a light-pollution rejection filter. When invoked for a given d-s object, the program calculates the sky darkness factor, film hypering exposure compensation adjustment, and bracketing range. It also shows an approximate estimate of skyfogging limit. None of this is relevant to today's CCD imagers, but it might be interesting to play with: the experienced film astrophotographer might also be curious to see how the program predicts values that he or she may very well know by practical experience.