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It is due to radiation loss (IR). The temperature of a dark sky is ?(3.5K) colder than your optics.
Hence the phenomena of frost when the ambient air temperature is 2 or 3 degrees on a clear night.
No frost on a cloudy night even when the air temperature is 1deg C.
This all depends on RH and Temp.
Microclimate is the major consideration. Even a slight local warming can stop condensation. The mechanism
depends on the design. All your scenarios are correct. Unfortunately there is no simple answer without ruining
seeing.
Even the smallest temperature variation can give you condensation or not. The other critical factor is nucleation
on the surface.
A very carefully worded question.
Bert
Bert
Last edited by avandonk; 15-02-2006 at 07:18 PM.
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