Re: Cooler than ambient?

Started by AlanFrench, 08/09/2004 09:59PM
Posted 08/09/2004 09:59PM | Edited 08/09/2004 10:02PM Opening Post
Dan,

Rick gave you some good information.

When you take a warm telescope outside on a cool night it immediately starts cooling. There are two basic mechanisms at work - convection and radiation. The tube warms the air next to it, which rises to be replaced by cooler air. The tube also radiates to the night sky. At first, convection cools more than radiation. As the tube approaches ambient air temperature, radiation takes the lead. Eventually, the tube will cool down below the ambient air temperature, and the warmer air will begin heating the cooler tube. At some point, you will get an equilibrium between warming by convection and radiational cooling, and the temperature will stop dropping. If this temperature is below the dew point, you will get dew on the tube.

Both cooling by convection and warming by convection are far more effective on a windy night, so dewing is far less likely if you have a breeze. The scope simply can not get far below the ambient air temperature.

I am surprised folks don't use fans to prevent dew. As Dennis di Cicco suggested long ago in a similar discussion, an umbrella would help keep amateur astronomers warm at night by cutting down on radiational cooling .

Clear skies, Alan
Posted 08/11/2004 01:04PM #1
If I understand this thread correctly, if you had a primary box of aluminum painted flat black, it would cool very rapidly, the primary and would always radiate heat to the box - this might aid in getting the primary close to ambient - but the box would probably have significant dew problems.

Are there other problems that a flat black aluminum primary box would cause?

Clear skies,

non alien Dan