dew removal

hey folks,
scopes have to be cooled so they will work properly,
why is it when you use a dew heater system it doesnt create problems with seeing,
if a scope is cooled down, then a heater strip will create heat,

It's a good question. I suspect that the mass being heated is too small for turbulence to be significant. If the whole telescope is warmer than ambient temperature then the amount of heat being transferred into the optical path would be greater and we'd have greater disturbance of the optical path. The mass of a secondary mirror is quite significant. But I really don't know.

A related question is why do we heat some surfaces - to prevent dew - but we cool the chip (with CCDs - not usually with webcams) and these don't usually get hit by dew (I know we cool them to reduce thermal electrons - but why aren't we encouraging dew?). Yet our sensor arrays are emitting infra-red to the coldness of the night sky just as our primary and secondary mirrors do (or their refractive equivalents) and are exposed to the same humidity.

What do the professional observatories do? They cool their CCDs alright (using liquid nitrogen from dewars) - but I don't know if they ever heat any optical surface.

I think its a good question.

Mark C.