Very basic questions

If I use a DSLR w/ a t-ring adaptor how do I calculate magnification? I use to use a coolpix 4500 and my meade super plossls. Now I moved up to a used rebel XT and I am just working this out...there is no EP so no magnification calculation (?).

Also, can anyone recommend a very basic book on using the Rebel XT?

The easiest formula is to divide the focal length of your telescope by 55 for a full frame 35mm camera, or DSLR. For most DSLR's which use a smaller chip, divide by 35.

Traditionally, it was thought that a 55 mm lens was "normal" since it covered the diagonal (55mm) of a 35 mm film frame. This gave the perspective one would get with the human eye--1 to 1, or 1 power.

However, the smaller DSLR chips only need about 35mm on the diagonal, and therefore a 35 mm focal length lens gives the same perspective (multiply 35 x 1.6, and you get 55). This is the source of the extra apparent "focal length" of a standard camera lens (designed for 35mm camera). You have seen that 1.6 figure before. In early DSLR days it impressed people that they were getting that much more focal length out of their lenses when they switched to DSLR's.

However, before this discussion goes too far--it should be said that there is really no single measure of "power" when it comes to discussing astroimages. (You have to settle on a definition of what the viewing distance is first.)

Alex