Brief update… yesterday, I opened the chip „chamber“, re-dried the desiccant tablets in the microwave and closed everything back up. When I had ordered the camera two years ago, my astro-dealer also recommended and sold me the dew heater for the camera. So I finally installed it as well.
Last night everything was fine, no more blotches or central dark spot/area. I also cooled the camera down in smaller increments, first to 0°C, than -10°C, later -15°C.
Tonight, I forgot the incremental cooling and the TEC went straight to -15°C. Immediately, the large „blotch“ was visible in the first Live stacks I did on 2017 K2 Panstarrs. … So I removed the camera from the scope and looked into the window. And indeed I could see a spot on the camera chip… due to frost/condensation. Confirming both, the experience from my Astro-dealer and the CN observations.
Back to re-heating the chip and trying to cooling it down more slowly.
