Quote:
Originally Posted by Camelopardalis
Regarding the offset...
..if you look at the histogram of a bias frame, it should show a nice bell curve, which will have a lower and upper extent, with the peak being the median.
The offset will translate (aka shift in one dimension) the bell curve along the horizontal axis. It's kind of like a pedestal. To maximise precision of your frame calibration, then adjust the offset such that the left-hand extent (lowest value) of the bell curve pulls away from origin (zero). If you think about this, then you want your calibration masters to fully, numerically represent what the sensor is outputting, since you will be subtracting this from all your data frames.
Like Greg says, this is like a black point. If you clip on the low (black) end, you can end up with precision errors, or depending on what software you use, unsigned integer rollovers (where negative values can end up represented as high values).
Note that my experience is from my ZWO camera, and it's possible that QHY handles this differently...but the fundamental principle should be the same.
To eliminate a potential source of error, I fix the offset (at 50 on my ASI1600) and vary only the gain. As an aside, I always use the same USB speed value also, since my camera does not have memory buffer, and reading from the camera at different speeds could potentially alter the read noise profile.
Regarding experimentation...I would suggest taking your experimental frames using the ASCOM driver. I have heard some hairy stories from a mate with a QHY camera and the "native" support in Sharpcap. (FWIW, I use Sharpcap a lot for planetary and lunar and it is a great piece of software, but it is still at the mercy of the driver with which it must talk to the camera)
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Nice explanation, thanks Dunk.
Greg.