Phil,
To my surprise, I've had a heap of condensation on the mount twice in the last month or so when it's been cold. I keep a cover over the scope and mount
http://www.iceinspace.com.au/forum/a...e.php?a=102717
with a few silica gel canisters around the mount, but what I think has happened is that some moist air has been trapped inside the cover when I've put the scope to bed, and the moisture in the air has condensed when the temperature has dropped. Unfortunately, once it's in the air-space inside the cover, it's trapped there and the small canisters are obviously no match for that quantity. I'm now considering how I might pipe some dry air up through the pier.
I also have a Delonghi dehumidifier, but it's more for drying out the observatory and other contents eg. books / papers etc. Having said that though, the other night I operated the dehumidifier for about 20 minutes around the telescope and mount before slipping on the cover. I'll see whether that worked next time I uncover it in the cold.
I have noted that the dehumidifier's heat exchanger tends to ice up if the ambient air temp inside the obs is much less than about 5 - 7 deg C. The Delonghi also needs a button pressed to start it (so a timer in the GPO won't do it); I intend to investigate its inner workings one day to get around this.
And finally, just to add to the radiative cooling discussion - on a clear night an infrared thermometer aimed at the zenith will typically indicate -30 to -38 deg C here. No wonder stuff cools quickly. In summer it is not quite so cold - at about -20 deg C.
Chris