For anyone following this thread, this project draws to a close. And none too soon.
I realise its not everybody's cup of tea - it's DIY and satisfying as its own end.
Cost - conservatively, including all components, not including the FET's I fried, because they were the wrong parts ~180AUD, which included the power supply. If you already have one, then less than a 100AUD should do it. The biggest investment, because it was a first of, is time.
All is working as planned, with a couple of improvements outstanding.
Good points - cooling can be controlled very well within a practical range of 30C below ambient, 34 if need be. It will cool further, but progress is slow and control is difficult because the system is at the edge of its performance envelope.
Cooling is fast at 100% pwm - seconds or minutes depending on the desired setting. The bigger the differential the longer the cooling. Alternatively, setting the pwm variable, for the camera temperature selected, slows the rate of cooling, which may be better if thermal stress is an issue - though I don't think it is - this is my preferred method, as it is much smoother.
Condensation - noticeably less when using live view. Dew control is a WIP and I'm looking at a chemical solution. Used sensibly, cooling to above dew point presents no problems. My view is, that because the cold finger is in direct contact with the back of the sensor, sensor temperature is not greatly affected by heating during long exposures - however, I have no way of testing this.
I did attempt a
first session and took some frames, including calibration. With 10 - 15C reduction from ambient noise is significantly less.
Dissapointments - It was necessary to employ a second Arduino board because cooling became unstable when multitasking - a consequence of my limited programming skills.
If anyone is writing a no delay shutter sketch with mirror lock-up, perhaps using the MsTimer2 library, I would like to hear about it. In any case the stepper that I use to control the GEM's hand controller pretty much stops everything else while it does its little 50 step dance.
Power supplies - Linear is better. Switch mode has been a pain with interference, requiring a fair bit of ripple smoothing to make it usable.
The heatsink is not mounted square and detracts from the appearance, in my view. Broke the golden rule on this occasion - "measure twice, cut once".
Was it worth it. Well I have to say yes. Truthfully, this project involved a very steep learning curve and I'm surprised by the results - better than expected, though it would be nice to build the electronics into a more professional package - time.