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Dew
Okay, that (secondary dewing up) is the more familiar pattern. While primary dew problems do occur, it's must less common. The primary - being larger and less "exposed" to the clear night air, will much more slowly radiate itself to a temperature below the dew point. It's also a tad more common (it's said and the science of radiative heat loss seems more consistent also) in truss designed scopes with a more exposed primary (no surrounding mirror box or tube to reduce the radiative heat loss).
Plus warming your primary is really the last thing you want to do, as that will impair image quality by causing distortion of the mirror and creating a boundary layer of warm air to layer along the front of the mirror.
One solution: as suggested, a dew shield will slow dew up by blocking radiative heat loss from the secondary mirror.
Another: as needed warming the secondary up with a dew blaster (or a small hair dryer). 12V products are out there like rmcpb suggested. If you have electricity at you site, I suggest a small 'travel' style hair dryer. Mine - purchased at Big W I think - has a "nose" just the right size to slide into a 2" focuser. I put it on the low setting, stick it into the (empty, obviously) focuser tube, and take a 4-5 min break while it warms the secondary and thereby removes the dew. Yes, it can contribute a bit to tube currents but that'll be better than not seeing at all.
Best solution is a low output dedicated secondary heater designed to warm your secondary mirror to keep it just barely above the dew-forming temperature. Many commercial products are available. This requires either routing power to the secondary (thin wires running along, or designed into, the secondary vanes) or getting one of the 9v options where the 9v battery actually mounts above the secondary so as not to block light to the primary. Hard to believe a 9v can warm anything, but I've heard many folks say they work.
WORST solution - wiping your secondary with a towel! You could scratch it and it will be dewed up again in 3 minutes....
Scott
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