andrea tasselli:
To be fair to ZWO they "kind of guarantee"a DeltaT of 35 degrees C which means that if your outside temperature (dry!) is more than that do not expect it can cool the thing to below zero. If the outside temperature is, say 25 degrees C @ 85% RH your dew point is 22.3 degrees C, so condensation will occur from any temperature below that. The main point to take away here is that as long as your dark library is up to date there isn't much to gain going below zero except that the thermal noise will go up (a bit) but in a gentle fashion (in fact gentler than some more modern sensors). Also, the dark current is pretty small so you don't loose much with exposures up to 300s (and depending on your targets, up to 600s).
Thanks Andrea, I believe you are right regarding those numbers. I was hoping that the problem is in the sensor rather than outside the camera window. So looking at the ASHRAE Psychrometric chart N2, that considers dry bulb temperatures below -10 one can guess the limit of the residual RH that is allowed inside, it has to be below 10%RH, even 20% should be fine, but if the problem is the external part of the front window...you are right, no way to avoit it, however the external heater should help a lot, and there are not unique results with that heater, some people claim that solves the problem but not always, so, it is a kind of trial and error.
