In English, “telescope” covers both mirror instruments (reflectors) and lens instruments (refractors). In French, “télescope” means only a reflector; a refractor is a “lunette”. The main families of reflector are the Newtonian, the Schmidt-Cassegrain and the Maksutov-Cassegrain.
Related terms
- Newtonian telescope
The telescope invented by Isaac Newton in 1668: a concave primary mirror at the bottom of the tube and a small flat mirror at 45° that sends the image out to the side, near the top. With no lenses it is free of chromatic aberration and offers a lot of aperture for its simplicity. It needs regular collimation.
- 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.
- Maksutov-Cassegrain
A compact telescope closed at the front by a thick, meniscus-shaped lens; the secondary is an aluminised spot on that lens. Its long focal ratio (f/12 to f/15) in a short tube makes it excellent on the Moon, the planets and double stars. It takes a long time to cool down.
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