[avandonk]

There is only one rule. You need to keep the temperature of your optics above the air dew point temperature.

Circulating air works fine if the air temperature is higher than the dew point temperature. When there is fog the ambient air is below the dew point.
Dessicant will do the job if there is enough in a sealed environment. It has to be regularly regenerated and is a pain to manage.

Supercooling is your main enemy at all times.

In Mikes case the observatory dome and to a lesser extent the walls radiate heat to a sky that can be far colder than ambient air temperature. This cools the observatory structure to below ambient air temperature. The contents in turn radiate their heat to the now cooler observatory dome and walls.

Condensation occurs when your optics temperature falls to or below the dew point temperature.


I heat my whole optical train to a set temperature that is thermostatically controlled. In summer 20C and in winter 16C. I never get condensation on the heated bits. This also means that focus does not change with ambient temperature. This is fine for a sealed optic.

In Mikes case I would put a dew heater strap around the tube near the primary mirror so the warm tube radiates heat to the surface of the mirror. This straps temperature should be controlled to be a few degrees above ambient air temperature. Same for the secondary and camera. The power needed is only tens of watts if you insulate the dew heater straps and the area being heated. I am going to get that space blanket aluminised mylar sheet that NASA uses on its spacecraft to replace the towel I am using. It reflects back 90% of the heat you would lose by radiation.


Heat will always go from a hot body to a cold one. To stop it or reverse it you have to apply energy in the form of heat or work.

Bert