Most accessories from one telescope manufacturer will fit on other brands, although counterweights are a notable exception. There are basically two systems of optical accessories; the ones that screw on to the thread on the back of a Meade/Celestron and the T-system, historically developed for the photographic camera industry. Adapters are available for changing between the two systems and coupling to cameras.

Changing the effective focal length

Once you have tried photography at the prime focus, you might find some astronomical images a bit small on the negative. The first approach to this problem is to purchase a tele-extender. This is a small negative lens which fits just in front of the camera and effectively doubles the focal length of the telescope. Since the camera sees a light beam from an f/20 telescope, exposures of extended objects will have to be about four times as long as at prime focus. The longer focal length also means a smaller field of view and thus precise focusing and atmospheric seeing become more critical. For longer exposures, the quality of guiding must also be higher or the star images will be trailed. For even higher magnifications, an eyepiece may be inserted into the light path to act as a magnifier. This is described in Chapter 9.

For larger astronomical objects such as the Orion Nebula, a wider field of view is desired and so a tele-compressor can be purchased.