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Hi Peter,
Are you using adaptive dark frames? I use these of a very long DF exposure and get negative numbers for some of the hot pixels.
What software do you use to take your calibration frames?
Flats will not change the sign of the minimum value as the operator is a division.
Attached are two images from a stack of images of C/2013 V5 that I took last night, apart from being two different images, one has been calibrated and one not so, you can see the hot pixel to the left and a bit below the comet? Its value in the uncalibrated image is 65k, and -512 in the calibrated image.
Doesn't seem to matter in the final image though.
Cheers
Stuart
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