A focal reducer (telecompressor or just compressor) is the opposite of a Barlow lens - it is a positive (convex) lens that makes the image smaller and brighter, reducing the focal length and f -ratio (Figures 6.8, 6.9). Because it has to go
20 or 30 cm in front of the focal plane, a focal reducer can only be used with Schmidt-Cassegrains, Maksutov-Cassegrains, and some refractors.
Meade and Celestron make virtually identical four-element focal reducers that convert their f /10 Schmidt-Cassegrain telescopes to f /6.3 while reducing field curvature and other off-axis aberrations. Although marketed as a photographic accessory, this device can also be used visually and works well with a 26- or 32-mm eyepiece for rich-field viewing, at far lower cost and bulk than a 2-inch-diameter diagonal and eyepiece. I have used both configurations and frankly prefer the focal reducer.
The one drawback of a focal reducer is that the telescope can no longer fill as large a field. If you start with an image designed to cover 35-mm film and shrink it, it no longer covers 35-mm film. Some astrophotographers find this bothersome; I just accept it as inevitable.
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