Hi,

I have a QHY9 camera and everything has worked fine a long time.
Now however I have found an odd problem.

I have offset about 1000 units in all my images.
I went trough them all in CCDStack and there is no zero values anywhere.
After I select Bias, Dark and Flat frames and do a calibration, as usually, CCDStack shows minimum value
about -700.
I havent been able to pinpoint the problem.

Many cameras have avg bias levels near 1,000 ADU and that's probably a normal value for an un-subtracted frame. That bias-level should become zero after dark-subtraction, though any sky glow will set the new base-level to the avg sky brightness (so unless the exps are very short or narrow filtered then you will not see a zero background level).

Minimum value is generally not very useful because it is highly affected by noise (e.g. read noise). To assess your avg background levels, draw a box around a dark region with few stars and then look at the median and mode (mode is the most common value and is thus unaffected by stars, unless they are very numerous).

If you are seeing an unexplained high value after dark subtraction then examine the dark and bias frames (and make sure the correct names are specified in the Calibration form). Also, if you use adaptive subtraction then pay attention to the reported factors, which should be near 1.0 for time/temp matched darks.

Note that camera firmware/driver updates can change the pedestal or bias levels and require new dark/bias libraries.

Stan