OK, my word isn't good enough, so here's an explanation written in nerdese: http://home.comcast.net/~astrophoto/Articles/Dew.htm It's on the Web, so it MUST be true. All you have to do is warm the optics above the dew point. That temperature varies dependant on humidity. You don't need "warm" optics to prevent dew or frost. In most cases, the optics are still cool to the touch. Mat is on the right track with his dew chaser project. Those of you who are in need should seriously consider attending his next ATM session. A properly designed dew chaser won't consume a lot of current, but will add hours to your observing/imaging sessions when the relative humidity is high. When my work schedule allows, I'll search the S&T archives for some articles they've printed over the years on active and passive dew/frost prevention. I think I get a few days off in late December. :-(