duncankitchin
Well-known member
Just wanted to write something up concerning an issue that I've been having (now apparently fixed) and what I think the cause was, and see if anybody else has seen this. In the process of trying to figure out what was going on, I found a lot of (old) threads related to SBIG cameras, negative PEDESTAL values and FITS files generated by TheSkyX. I happen to be using an STL11k and TSX...
The issue first showed up when re-processing some data that I had from last summer's Oregon Star Party. This had previously been processed without issue, but when I reprocessed I found the calibration was very wrong. Specifically, the flats were all messed up and "over correcting", that is, I had white corners in the calibrated lights. This was very odd, since I have a well-debugged PI calibration workflow that I've been using without trouble for years, including on the exact same data. Eventually I worked around the problem temporarily by going back and pulling my master flats that I built last year from the same data.
They worked fine; but my newly generated master flats from the exact same data and created using the exact same process steps were no longer working. I have been trying to figure out what is going on for weeks...
It turned out to be an issue with the new behavior in handling negative PEDESTAL values, in that by default it is accounted for in calibration, when previously it was just ignored.
It happens that just ignoring it in all frames works fine with my calibration flow - it just comes out in the wash as an additional component of bias. It would also have worked fine if it was accounted for in integration as well as calibration, but ImageIntegration seems to still ignore it.
My process is to directly integrate bias and darks to get a master bias and dark, and then calibrate the individual flats. That way, when I do an integration of the flats, they are already zero-based and I can therefore make use of multiplicative normalization and "equalize fluxes" for rejection normalization. When I do light calibration, I just need to be careful to set the "calibrate" checkboxes appropriately to account for the fact that the master dark has not been calibrated but the master flat has.
My process is all working again now - but I need to set the pedestal mode to "literal value" and set the value to 0 when calibrating both flats and lights.
Has anybody else seen this, and does what I'm seeing sound correct?
The issue first showed up when re-processing some data that I had from last summer's Oregon Star Party. This had previously been processed without issue, but when I reprocessed I found the calibration was very wrong. Specifically, the flats were all messed up and "over correcting", that is, I had white corners in the calibrated lights. This was very odd, since I have a well-debugged PI calibration workflow that I've been using without trouble for years, including on the exact same data. Eventually I worked around the problem temporarily by going back and pulling my master flats that I built last year from the same data.
They worked fine; but my newly generated master flats from the exact same data and created using the exact same process steps were no longer working. I have been trying to figure out what is going on for weeks...
It turned out to be an issue with the new behavior in handling negative PEDESTAL values, in that by default it is accounted for in calibration, when previously it was just ignored.
It happens that just ignoring it in all frames works fine with my calibration flow - it just comes out in the wash as an additional component of bias. It would also have worked fine if it was accounted for in integration as well as calibration, but ImageIntegration seems to still ignore it.
My process is to directly integrate bias and darks to get a master bias and dark, and then calibrate the individual flats. That way, when I do an integration of the flats, they are already zero-based and I can therefore make use of multiplicative normalization and "equalize fluxes" for rejection normalization. When I do light calibration, I just need to be careful to set the "calibrate" checkboxes appropriately to account for the fact that the master dark has not been calibrated but the master flat has.
My process is all working again now - but I need to set the pedestal mode to "literal value" and set the value to 0 when calibrating both flats and lights.
Has anybody else seen this, and does what I'm seeing sound correct?
. I have everything carefully archived because I frequently go back and reprocess old data.