Rob - you're on the ball!
A fan is really only useful for equilibrating your OTA to ambient temps, otherwise it's just blowing more damp air around!
A mini (12V) hair dryer will help keep your optics clear, but use lots of battery juice rapidly.
I had a near-disaster camped out above the Murray at Swan Reach:
"I'm with Alexander: drove out to dark site to camp out overnight on a cliff high above the Murray River, intending to video the 2012 Transit of Venus C1-4 contacts.
The next such transit isn't until 2117, so last chance!
I had no idea that so much moisture would arise from the river: in the morning, just before the start of the C1+C2 transit events, my dew heater battery gasped its last & through the streaming water, I couldn't make out the Sun, let alone Venus!
(Got C3+C4 later in the afternoon as the sun dried everything out: phew!)
http://www.iceinspace.com.au/forum/s...ht=worst+stuff