Astronomy astro-images and beginner guide | Tutorials on choosing telescope and related equipment
A Focal Reducer effectively reduces the focal length of the optical system of the telescope so that for any given eyepiece a wider field of view will be attained. The Focal Reducer is useful in viewing or imaging an object that would otherwise would not fit in the native field of view of the telescope or telescope-camera system.
Focal reducers are very useful for astrophotography in that the field of view is not only increased but that the focal ratio of the optical system is diminished, meaning a shorter required exposure time. They are used for stars and DSO and are not used in lunar or planetary viewing or imaging. They are excellent for helping match image scale and pixel size. This is especially useful for Schmidt-Cassegrain telescopes and also used with refractors.
Some focal reducers also act as a field flattener to significantly reduce the field curvature in all Schmidt-Cassegrains and most refractors that causes stars to be out of focus at the edge of the field when the scope is sharply focused at the center.
Before buying a Focal Reducer make sure your telescope has enough inward focus. Also the Focal Reducer is not useful in telescopes such as the Maksutov-Newtonian which is designed to operate at a certain focal ratio to produce pinpoint stars and a flattened field.