Hi Bernd - Thanks for your response. Given that I just started diving into PI, could you provide a little more specific procedure I would use to follow up on your suggestion? I have no idea of what a pedestal is and not sure how to check clipping with HT and Statistics.
I will cite a section of my
guide about this topic:
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8 Checking the Calibration Result
8.1 Checking for clipping in the low range, Output pedestal
After following the above suggestions, you should test some of the resulting calibrated light frames for clipping in the low range
- with HistogramTransformation:
check the histogram (adjust horizontal zoom that the histogram region around intensity 0 can be inspected carefully, this might be the case at a horizontal zoom of about 50 - 100) and
- with ImageStatistics (option 'Unclipped' disabled):
check count (%), it should be very near 100 %. The fraction of clipped pixels equals (100 % - count (%)).
Following the calibration approach recommended in this guide, the calibrated light frames normally should not be clipped. If you nevertheless detect clipping, the calibration result is not correct, so please check the following conditions carefully: acquisition of calibration frames, preparation of the master calibration files and settings for the calibration of the light frames. If you cannot find a cause for it, clipping is possibly caused by a too low signal in the light frames (e.g. due to very short exposure time or the usage of narrowband filters and short exposure time). If the noise in the background exceeds the median of the background in the calibrated frame, some pixels will have negative values after image calibration.
When this is the case, things cannot be put right by increasing the offset setting in the camera driver, because this would affect light frames and all calibration frames in the same way. The purpose of offset [2] is to prevent clipping of individual subframes during A/D conversion.
Clipping due to the subtraction (light - MD) performed in the calibration process is prevented by applying an
output pedestal in the calibration of the light frames. If needed, the value of 'Output pedestal (DN)' has to be set in the 'Output Files' section of the ImageCalibration process. Usually a value in the range of 100 - 200 DN is sufficient. It doesn't hurt if the value is a little bit larger than necessary for avoiding clipping though. Again the new calibration results should be checked as described above in order to make sure that the value of the output pedestal was sufficient.
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Bernd