Astrophotography Notes

The astrophotography calculator is intended to be a simple aid for anyone who wishes to take photographs through their telescope or camera lens at celestial objects. Neither the calculator nor these notes can replace a good book on astrophotography or experience gained from getting out with your telescope, film, camera and a note pad. None the less the following points are of enough importance to emphasize.

Do not take photographs of the Sun without using a securely fastened full aperature solar filter (or a stopped down solar filter mounted in a cell in front of your telescope). Adjust the Sun's brightness by the light transmission percentage of the filter, and factor in the aperature stop (if any) to the primary diameter.
Take notes of every photograph you make - this is the only way you will learn how to improve your skills. The output list generated by the calculator provides much of this information automatically, but you should add the film brand and type to the notes.
Bracket your exposure times. Use exposure times shorter than, equal to, and greater than the one provided by the calculator. Record the speed for each frame.
Beware of camera induced vibrations or limitations.
- If you use an SLR, lock the focusing mirror mechanism up if possible.
- For exposures longer than a fraction of a second (or if you can not lock the focusing mirror up), place a black card in front of your telescope (large enough to more than cover the tube, but don't touch the scope), lock the shutter open, wait for the vibrations to stop, then move the card away from the tube for the length of the exposure, move the card back in place, then unlock the shutter (you will probably need a cable or airline mechanism to lock the shutter open and minimize vibrations).
- Beware cameras with electronic shutters: these use the main camera batteries to hold the shutter open, and only a few moderate length exposures will drain the batteries.
Make sure all locking screws and threaded adapters between the focusing unit, eyepiece and camera are tight.
The longer the exposure time, the more chances of something going wrong.
Beware empty magnification with eyepiece projection (it's both easy and tempting to push past the limits of your telescope and the current atmospheric conditions): the temptingly larger image at f/100 will probably not yield any additional detail, but it will increase your exposure time -- if you want a larger image (with no increase in detail), enlarge it from the negative.

The calculator does not factor in reciprocity failure of film into the exposure times, because each film fails differently depending upon many factors. For exposures more than a few minutes you should try gas-hypered films to minimize this problem.

Prime Focus

                +---------------+
camera ->       |               |
                +----|     |----+
                     _______
T-ring ->            !     !
                    =========
                      |   |
camera adapter ->     |   |
                      |___|

Apply any focal reducers or extenders to the Primary FL in the calculator.

Eyepiece Projection


                +---------------+                -
camera ->       |               |                 |
                +----|     |----+                 |
                     _______                      |
T-ring ->            !     !                      |
                    +++++++++                     |
                    |       |                     Y = projection length
                    |       |                     |
                    |       |                     |
Eyepiece            |       |                     |
 projection ->      |       |                     |
 adapter            | __~__ |                    -
                    | ##### |  <- eyepiece       -
                    | ##### | (inside adapter)    |
                    | ##### | with focal length   X
                    |       |        f            |
                    =========                    -
                      |   |
                      |   |
                      |___|

Y is the distance from eyepiece to film (camera back).

X is the distance from the eyepiece to the focal plane of the primary. It can vary between f (focal length of eyepiece) and 2 * f: this distance determines Y. If you have a fixed length adapter, then Y will fixed for any given eyepiece and will determine how far out or in you must focus the projection system to get an in-focus image at the film plane.

Simply put, the closer to f X is, the longer Y will be (and the larger the resultant image will be -- at X = f, the image size on the film will be infinity, as will Y), and the closer to 2 * f X is, the shorter Y will be (and the smaller the image size on the film will be -- at X = 2 * f, the image size is the same as if no eyepiece was used at all).

For a fixed length adapter, Y will only vary if the top of each eyepiece extends out by a different amount (a physically taller eyepiece will reduce Y while a shorter eyepiece will lengthen it) or if you use cameras with different distances between their film planes and the T-rings.

The magnification of the optical system = ( Y / f ) - 1

For a variable length adapter you can vary Y, which also allows you to vary X, but the calculator does not currently allow you to specify a value for X (the magnification of this optical system = Y / X, where f < X < 2 * f), so assume that you can only modify Y and adjust the focus accordingly.

Equations are from Astrophotography by Barry Gordon, Second Edition, 1985, Willmann-Bell, Inc. (ISBN 0-943396-07-7).

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