Good articles from Sky & Telescope.

Dave if it's just a telrad you could try wiping it with that anti-fog stuff you can get for the inside of car windows. I think you can get it from halfords. Just rub a teeny tiny bit in and no more foggin. It repels water.

You'd only need to rub it in once every few months or so. Seems like a lot less effort than adding a dew heater.

Dave if it's just a telrad you could try wiping it with that anti-fog stuff you can get for the inside of car windows. I think you can get it from halfords. Just rub a teeny tiny bit in and no more foggin. It repels water.

You'd only need to rub it in once every few months or so. Seems like a lot less effort than adding a dew heater.

I've tried similar products on my Telrad with no great joy. It doesn't take much to make the faint sky background a blurry mess if the window isn't absolutely clean and dry. So far I get the best results using a long cardboard dew cover for the Telrad... but that has to be replaced regularly, and it still dews up when pointing close to the zenith I think Dave L's on the right track with the resistors - might try it myself - besides, a dew heater looks cool!

Well, I got the resistors today and will be putting it together over the next few days, I'll put up a new thread when its done to let you know how it went and what's involved.

Thanks DaveG for the tip, although I wont be going down that route at this stage.

I used the anti-fog stuff before and as long as the surface you
apply it to is 100 pc clean and grease free it'll work fine.

Dave,
Overkill is the only way to go....
at least you know for sure its GOING to work!