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It is definitely caused by the power supply and some sort of capacitive coupling. I have not posted the results yet but I am on the verge of solving the issue, at least with my camera.
1. I swapped the humidity and TEC control pins by some soldering on the board.
The humidity pin can use hardware PWM but the original TEC pin cannot. This will allow us to do faster TEC switching.
2. At 8KHz switching frequency I could not notice any "noise". The frequency is higher than the line reading frequency.
3. Another improvement has been achieved by using electrically conductive thermal paste between the cold finger and the back of the sensor. This puts the cold finger at the same potential as the back of the sensor and it helps a lot.
DO NOT GROUND THE COLD FINGER.
I am still doing tests but it looks like this has solved the problem for me. Few other people will be doing tests soon and I will post the results then.
If needed further options would be to connect ground directly from the power connector to the VT1 MOSFET. Possibly even move the VT1 from the main PCB. Optoisolator would probably help as well.
Do not use a single stage TEC, you won't get very high temperature differential. Quite a few of us used TEC2-19003 from ebay and it is possible to get 40 degrees C temperature difference.
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