[avandonk]

Yes any object that is exposed to a hemisphere of cold dark sky will cool down lower than ambient air temperature. That is why frost occurs even when the ambient air temperature is above zero C on anything out in the open. The so called dew shield on your scope only slows down this effect by limiting how much cold sky your telescope 'sees'.

The mechanism is quite simple it is due to heat flowing from a hot body to a colder one by radiation. It is called supercooling.

A clear night sky can have a temperature far lower than ambient air temperature at ground level. So anything exposed to it will equilibrate at a temperature lower than ambient and if that is below the dew point you get condensation.

An uninsulated observatory roof will also cool below ambient and then start to cool what is under it below ambient.

It has not alot to do with ambient air temperature just the dew point and the supercooling of any object due to radiative heat loss to it's environment be it a cold sky or a cold observatory roof or dome.

I used to thermostatically heat my 300mm lens to about 16C in winter and 20C in summer 24/7 to stop condensation and focus shift. I am currently heating my RH200 to 17.0 +- 0.1 C so focus does not change and there is no chance of condensation. You would be amazed what nasty compounds are in the air from plants, animals, your observatory structure and yes you.

That nice blue air haze in the mountains is due to compounds from all the gum trees. Some are resin like.


Bert