Something odd happens during processing I’ve experimented with for photometry involving Canon DSLR CR2 RAW images, IRIS and Astroimage J (AIJ). I’d be interested to know if anyone can explain it.
I find AIJ convenient for photometry on monochrome images taken with a ZWO ASI1600MM camera and Johnson photometric filters. I would therefore like to try AIJ on colour images taken with a DSLR camera. As far as I know, that requires Canon DSLR CR2 RAW images to be converted to the FITS format, debayered, and the G and B channels to be separated (mostly, I don’t process the red channel). I’ve been undertaking these steps, apparently successfully, with IRIS.
The images created in IRIS and imported into AIJ are all FITS files (.FIT extension): science images (green and blue channels), individual flats (same channels) and individual darks (any channel seems to do, as the ADUs are the same for all three).
AIJ creates a master flat without any problems. However, the counts in the master dark are very different from those in the individual darks. Stats on one of the darks, for example, are: Minimum 0 (zero); Mean 1023.011; and Maximum 15,760 (hot pixels). The same stats on the master dark are: Minimum 32,768; Mean 33,790.995; and Maximum 48,528. That is, 32,768 has been added to the image during the creation of the master dark.
The issue is easily resolved in AIJ by using Process > Math > Subtract, subtracting 32,768 from the image, then resaving.
The questions are: why does this happen? Can it be prevented?
Ok, something just triggered a memory and I went looking for the FITS keyword BZERO.
Perhaps it has something to do how AIJ handles this. I'll dig a bit.
There are much bigger problems. There seem to be inconsistencies between IRIS and AIJ. I'd run into these before.
I'm abandoning the project.