Ok I have had same issue too,
ambient temp of around 26C
setting cooler directly to -10c instantly started to frost over sensor.
basically if the whole unit is at or near ambient temp ( high temp days), the shock effect on sudden cooling of just the chip inside cause whatever moisture etc is inside to form on sensor.
the same occurs if you suddenly take it from -10 to say -20 mild frosting occurs again.
I have done this test over last two days.
I think that you need to do a cool down to say 5deg for about 10mins, then a cool down to 0 def another ten minutes.
then cool down to -10.
I notice the sensor will frost over partially with each step. but sublimes away after a few minutes.
I don't think the dew heater will do any thing towards stopping it
I have yet to do these tests when camera is already cool (ambient wise)
will follow up again.
I also added the AUX plug, which did not change it much
Another point is it may take a few cycles to absorb the moisture in the chamber.
Frosting will always sublime off to another part, usually called a cold finger.
another trick could be to flush the chamber thru the plug opening with inert gas, and reinstalled aux plug.
It takes magnificent photos once sensor is clear of frost.
