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.
Related terms
- 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.
- Focal reducer
The opposite of a Barlow: a lens that shortens the focal length, for example ×0.63 on a Schmidt-Cassegrain, taking it from f/10 to f/6.3. The field widens and photo exposures get shorter (2.5 times here). Many reducers also flatten the field.
- Dew
The moisture that settles on optics when they cool below the dew point by radiating heat to the sky. It strikes exposed surfaces first: Schmidt corrector, refractor lens, finder, eyepieces. Do not wipe it: a dew shield or a dew heater prevents it, a hair dryer clears it.
- Central obstruction
The fraction of the aperture blocked by the secondary mirror, as a percentage: a 50 mm secondary in front of a 200 mm primary is 25%. It does not make a hole in the image but slightly lowers the contrast of fine planetary detail. Visual Newtonians aim for 20 to 25%; Schmidt-Cassegrains often exceed 30%.
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