With the advent of digital cameras, mostly having fixed lenses, the focus procedure described below applies again.
This can be done in two ways:
Exposure = ------------------------ sec.
K x ISO film speed
Where: K is 20 for a crescent Moon
40 for first/last quarter Moon
200 for the Full Moon
13.6 for Saturn
32.5 for Jupiter/Mars
1310 for Venus
System focal length
and (f-ratio) = --------------------
System focal length = Camera lens focal length x Telescope magnification
Telescope focal length
Telescope magnification = -----------------------
Eyepiece focal length
This might look complicated, but most of the values become constants for a particular telescope and need only to be calculated once.
As a worked example, take the following which gave me the correct exposure before: My telescope has an aperture of 220mm and focal length of 1420mm. Doing afocal projection on the first quarter Moon through a 19mm eyepiece with a standard 50mm lens and 100 ASA film.
Telescope magnification = ------- = 75 times
System focal length = 50 x 75 = 3750mm
f-ratio = ----- = 17
17 x 17
Exposure = ------------- = 1/14 sec.
40 x 100
Setting the camera to 1/15 sec. and the lens to full aperture gave me the correct exposure when compared with exposures bracketed on either side of it as can be seen here from the picture.
To decide what focal length eyepiece to use, simply set up the telescope and camera and have a look to see what field of view each one gives. In general the lower power (longer focal length) ones work better.
Download a handy little computer program by Eric Smestad, based on these formulae.
Source: The exposure calculation formula first appeared in Astronomy Now, October 1992, p.18 in a slightly different form, with only the full Moon constant given. The values for the other lunar phases and planets were transformed to this system from graphs published in the 1959 book; Astronomical Photography at the Telescope by Thomas Rackham.
Also visit this page for similar content.