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With this little test rig, the temp differential ran to ~20C with a 12V 6A TEC and 4.8V applied to the 7R7 sensor dew heater, which is hot to touch, but not burning. I can live with that. Cold finger temp was -2.15 ambient 17.7.
I dropped the dew heater voltage to 4V - at -1.5C the sensor started to fog up. Increased to 5V, clearing in 5 minutes, with an increase of 1.5C cold finger temp. 4.8V seems to be the spot, evaporating light introduced condensation in about 5 - 10 seconds.
Given that this was an open air test, defogging should be more efficient with the sensor in the confines of the mirror box/rear telephoto lens.
For me - Myth Busted. Let's see how it works in the field on a cold night - might need to wind the TEC voltage back a little. -5C is adequate. Watch this space.
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