Last night I had an interesting dew problem that I wasn't expecting.
The setup:
GSO RC10, no dew shield, no dew heaters, cooling fans always on (I leave them running pushing air from the back to out the front on the theory that the air movement should stop the dew settling).
This is what I normally do and so far it's been ok, even on other dewy nights.
Last night however, the primary mirror (and not the secondary!) started fogging up. I hadn't noticed this happening before.
Given that there was nothing more unusual about where I was pointing in the sky to any other (dewy) night, I'm trying to work out what happened.
1. It's possible that on other dewy nights I might not have noticed this and had other problems I was working on distract me.
While it's possible, I do remember explicitly looking on occasion when there's been bad dew and not seeing any problems on the primary mirror (or secondary).
2. Maybe it was really heavier dew than I had so far encountered and the airflow from the fans wasn't enough to counter. Everything was very wet last night, but it didn't strike me as unusual from any other dewy night. However I only have subjective data here, so I can't say for sure.
3. The cooling fans cooled too much reducing the surface temperature too low.
4. Something else.
As the dew made it to the primary mirror, I can't see that a dew shield will help. If it's the fans, I can try turning them off and see how that goes.
Any thoughts?