(meteorobs) Heat required to prevent dew in a box for allsky camera

[rmbehr at istar.ca]

Koen, I'm interested in the heating method you use in the box for the  
allsky camera to prevent dew.  During summer I often put my camera  
(old Canon Rebel XT with wide angle lens) on a tripod and set it  
outside for a couple of hours hoping to catch meteors, and have been  
successful a few times.  I live in Ontario, Canada, so the summer  
season is short.  I have made a small box with a window that can mount  
onto a tripod, that has worked well.  I was thinking of building some  
kind of box with some kind of heat that will prevent the dew from  
steaming up the window and keep the camera above freezing.  I'm not  
knowledgeable when it comes to electronics, I was thinking along the  
lines of building the box big enough for a small heating pad, and have  
the camera on a shelf above the pad.  How much heat is usually  
necessary?

Any thoughts anyone?

Rose-Marie Burke