pleiades However, the cluster seemed vanishing during the night probably because of the signal to noise ratio.
For them to be tiny specks like that, and then going away, it is very likely to just be ice on the sensor.
The usual way to avoid it is to cool the camera down slowly as you lower the temperature. Take the cooler temperature down 5ºC at a time and pausing a few minutes at each step.
The root cause is the moisture in the camera chamber, and it may be time for you the replace the dessicant tablets.
However, if slowing down the cooling each evening cures the problem, it is better to leave the dessicant alone until you absolutely have to change them, since reassembling and avoiding dust is a pain (an illuminated sensor loupe, a hand blower with a HEPA filter, and sensor cleaing kits are almost mandatory to avoid dust after you opened the camera chamber).
Chen
