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Originally Posted by Dirk
This might be a silly question! Why is the temp of the mirror in telescopes so critical? Dose it have to be at the same temp as the outside air or can it be kept at a constant temp warm with a heater?
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There are two reasons why a mirror should be brought to very close to ambient (outside) temperature. As the other posters have pointed out thermal currents in the optical tube (aka tube currents) cause image abberations, however there's a secondary and just as important reason that is overlooked by many amateur astronomers.
The secondary reason is to cool the glass down so it stops expanding and contracting due to the influences of temperature changes. Mirrors and lenses expand when it's hot and contract as the temperature cools. The amount of expansion and contraction varies with the matieral lenses and mirrors are made of. The rate of expansion per °C is known as the thermal co-efficient of expansion. Plate glass for example expands many times more than Pyrex which in turn is worse than Zerodur/Duradur or Aerosital which have co-efficients that are for all purposes essentially zero for most optical purposes.
The typical change in a mirrors 'shape' may be only a a few millions of an inch from hot to cold, yet it is sufficient to wreak havoc on the image. How the mirror or lens is held, it's thickness and the type of optical design all have bearing on the way the optical element behaves while cooling down.
Finally optical surfaces may need to be heated to prevent dew formation, it's typical to aim for a temperature of about 1°C above the dew point as this will prevent dew forming. Such a small temperature rise generally won't change the optics sufficiently nor induce tube currents in a well designed tube.
