|
Under clear skies radiation will never reach equilibrium (not unless your scope's in outer space and cooled to about 3 kelvin) and that's why you get dew: because your scope radiates heat out into space and becomes cooler than the surroundings. (There is a thermodynamic tug of war between convection and radiation both preferring equilibrium, i.e., maximum entropy.)
Ideally, a dew heater would just make up for the heat lost as radiation and keep the optics at ambient temperature. In practice though it needs to bring what it's in contact with to just a smidgen above ambient so heat can flow to the rest of the system. With a good heating + cooling setup the image quality won't suffer: the job of heaters, coolers, fans is after all to get the best performance possible out of your scope. Best way to learn how is to hang out with experienced astro-imagers and watch them at work.
|