A 90° mirror or prism placed between a refractor (or Schmidt-Cassegrain) and the eyepiece, so you can look near the zenith without straining your neck. The image is then upright but reversed left to right, as in a mirror. Keep this in mind when reading a chart.
Related terms
- Refractor
An instrument whose objective is a group of lenses at the front of a closed tube. With no obstruction it gives very contrasty images, cools quickly and almost never needs collimating. For the same aperture it is harder to make and longer than a reflector.
- Schmidt-Cassegrain
A compact telescope closed by a thin corrector plate, with a spherical primary and a convex secondary, usually at f/10. Versatile and portable, it often comes on a GoTo mount. Focusing moves the primary mirror, which can shift the image slightly.
- Eyepiece
The small interchangeable magnifier you look through; it sets the magnification. Its focal length is engraved on it: the shorter it is, the higher the power. The standard barrels are 1.25 inch (31.75 mm) and 2 inch (50.8 mm) wide.
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