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.
Related terms
- Seeing
The restlessness of the atmosphere that makes images dance and boil; it is also what makes stars twinkle. It is measured in arcseconds (the size of a star in a photo) or on scales from 1 to 10 (Pickering) or I to V (Antoniadi). It limits high magnification far more often than the instrument does.
- 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.
← The whole glossary