Originally posted by DeadJohn: because the front objective gets colder than the air by radiating heat to outer space
I've been studying the last couple of days on the subject, and wow I didn't realize just how much radiative cooling could drop the temp of the glass below ambient!
Based on this I think I will make my own heaters using Nichrome wire, Velcro strips and some inexpensive PWM 12v controllers. While desiccant packs might help some on the 6" refractor, I'm thinking that they'd be almost useless on the 10" schmidt-newtonian due to the surface area of the glass. I already put a micro-particle air filter on the back of the mirror cell, and this virtually stopped the dew on the primary mirror, but the front glass, even with the big hood still condenses moisture like crazy.
Here's the S-N in action at a recent star party:
JB Media | 20141108 TAS Frisco Starfest
It may be early next month before I get started on the heaters, but I'll be sure and post the results. Thank you everyone for the tips and suggestions. Much appreciated!