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.
Related terms
- 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.
- Cooldown
The time a telescope brought out of a warm house needs to reach the air temperature. While the mirror is warmer, currents of warm air rise in the tube and blur the image. Allow 30 minutes to an hour for a 200 mm mirror, more for a Maksutov; a fan speeds things up.
- Focal ratio
The focal length divided by the aperture, written f/5: a 200 mm telescope with a 1,000 mm focal length works at f/5. A “fast” instrument (f/4 to f/6) gives wide fields and short photo exposures; a “slow” one (f/10 to f/15) suits the planets. For the eye, at the same magnification, it does not change how bright the image looks.
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