After cleaning the sensor again, I tried again last night and lasted only one sub before the sensor fogged over again. This is very frustrating, since I took great care to seal the camera and purge it with argon. It's becoming clear to me that without a cover glass over the sensor (effectively double glazing) the sensor and the cooling running at 2C - it just becomes a dew generator on humid nights.
All colour sensors in Canon DSLRs, and other brands, have a cover glass over the sensor surface. You can't tell it's there but what you see when you look at the sensor is the coverglass first and the sensor surface below that. It's there to protect the sensor, and supports the 'shaker' dust cleaning devices. The cover glass might have a coating to discourage dust attachment (anti-static coating).
When CentralDS debayered my sensor they had to remove the cover glass to do that work. CentralDS do offer to replace it with the stock (highly reflective cover glass) or to put on a Astronomik MC Clear cover glass, at additional cost of course. I opted to have them just send me back the bare debayered sensor and I was intending to use my Astronomik MC Clear EOS Clip-In filter. Well a MC Clear Clip-In does provide dust protection, it does nothing in terms of condensation double glazing.
So for now I am packing this project away for a couple of months. I have a EOS filter drawer on the way from Teleskop-Express, and I need to buy the small Astronomik MC Clear glass that fits into the sensor filter frame - and allows me to create a double glaze chamber just over the top of the sensor; I believe this may solve the condensation problem. If not I may have to get to a cover glass heater, which Roland has developed.
Just more development problems to deal with, I guess I will get there eventually.
