A
astropixel
Guest
Perhaps the light frames are under sampled in places or the superbias is faulty. It is a 200 frame master bias. I do not have access to the superbias module, with the latest v1.8.
Unless you have a space telescope, it is likely that even the darker parts of your sky is somewhat brighter than the plastic cap of your telescope (in fact, even with HST, as witnessed by the Hubbles Deep Field) . The bias value was probably selected by the manufacturer so that there is no negative value even in the noisiest darker part of an image (even without stacking, as most people do not stack their holiday pictures), unless we do something very extreme (which we may the way we use DLSR). So subtracting bias from an image should usually not cause negative values, but this may depend on the DSLR and other parameters.Hmmm... if I subtract a master bias from a dark frame I do get some zero value pixels. If I subtract a master bias from a light frame I don't get any zero value pixels - there are some pixels with very low values but no zeroes.
Well! I did just that and frankly, the best results came from a master_dark_with_bias and flat - just the same as the -5C set. The integration scaling factors and weights were ~1 and better, 1.0 - 0.98 at times. No observable evidence of truncated data.The same calibration/preprocessing exercise could be conducted using a camera that is not cooled. It would be very interesting to compare the results.
Sorry, but I fail to see how this can be possible.BITD prevents you from a much better read noise calibration of your lights
bitli said:Sorry, but I fail to see how this can be possible.BITD prevents you from a much better read noise calibration of your lights
If you subtract the read pattern (an unwanted signal, not a noise) from the dark, then you decrease its effectiveness in correcting the read pattern from the light, you do not increase it. The read pattern is already part of dark, even if noisy. Maybe I missed something or some steps in your process?
It may be that it is better to use a non noisy bias instead of a dark and all you want is to correct a read pattern - but I have not way to test that method as my darks are different from my bias and I need them. This however depends on the DSLR.
-- bitli