It seems unlikely to me that you have dew collecting on the CC but not on the secondary or the primary first. Or is because those are heated and the dew is so high that it would condense about anywhere? Still, even when I had similar conditions I never had it on the corrector as it was well tucked away within the focuser tube and the tube wasn't exposed to the sky.
Neither are heated at the mo. The primary was definitely fine and seconary looked okay from where i was looking but i could see the misted up CC through the secondary. I've bought the scope inside now so everything has condensed otherwise would of tried to snap a pic. I also don't want to fiddle with the optical train either as need to do flats tomorrow for the few hours of data i did get.
Andy Wray:
A related question: I have never used a dew heater on my Newt and have rarely, if ever, had a dew problem. The outside of the scope and everything around it ends up dripping wet with dew most nights, but the mirrors and the optics tend to stay clear. Why is this? @Michael Bate I have the same OTA, coma corrector, filter wheel and filters as you FWIW.
Neither have I up to tonight. The scope, filter wheel, camera casing and any other surface exposed was frosted but the raw image got gradually more blurry and there was definitely no cloud at that point.
Maybe i need to try snap a pic or pull the imaging train apart if it happens again and physically check the CC.
