There are several ways to take an astrophoto with a telescope. The following are the various optical combinations, starting with the widest field image:
Piggyback - the camera is mounted on the telescope tube and aimed in the same direction; the scope is used for guiding on the stars for long exposure, wide-field shots using just the camera lens.
Prime focus - the camera is aimed through the telescope, without a camera lens or eyepiece.
Negative projection - same as above but with a Barlow lens in the telescope.
Eyepiece projection - an eyepiece is in the telescope, but without the camera lens.
Afocal - an eyepiece is used with the telescope, along with the camera lens; this provides the most highly magnified image.
Taking a prime focus image can present a problem for Newtonian telescopes, which are normally optimized for visual observation, with the focal plane close to the drawtube. To take an in-focus image, cameras used in the prime focus mode require the focal plane to be about two inches beyond the focuser, which is beyond the range of most Newtonians. The usual solution is to move the primary mirror up the tube by the two inches needed for the camera, which may also require substituting a larger secondary mirror to handle the larger light cone. This reduces image contrast for visual use, and requires the use of an extension tube to focus eyepieces. This is the major reason why Newtonians are normally optimized for visual use; optimizing for photography reduces visual performance. Most through-the-telescope astrophotos are taken using either Schmidt-Cassegrains or refractors, which have longer ranges of focus. The alternative for Newtonian users is either negative projection or eyepiece projection. Both of these yield a narrower field of view and make the exposure more sensitive to guiding errors.Published in the May 2002 issue of the NightTimes