Hi
while recently laid up, I tidied up some hard disks and found about 3 hours of quite good Helix neb subs that had not been processed. However the data was taken with a combination of gain/offset/temp/USB/time that I did not have darks for, so I could not dark cal it without a trip to my imaging site (where the camera is). CMOS chips can benefit from dark cal to tidy up slight fixed pattern irregularities in narrowband backgrounds.
Rather than wait until I had real darks, I tried dark scaling and got some pretty good results. The subs in question were 5 minute Ha and the closest darks (with the same gain/offset/temp/USB) were 10 minutes. The 10 minute master dark was manually scaled to 5 minutes in PI using pixelmath, to make sure that there was no unwanted optimisation. [masterdark5 = masterbias + 0.5*(masterdark10 - masterbias)].
The images in
http://astrob.in/306733/0/ show the effect of the scaled dark - for illustration, the subs were stacked without alignment (so the stars show the dither pattern and the object is smeared out - but the fixed pattern noise is clear). Both uncalibrated and dark calibrated stacks (STF stretched) are shown for comparison. The two left hand panels show the stretched stack outcome with no dark cal - the faint glows typical of the 1600 are clearly visible, particularly in the lower crop panel which shows part of the bottom right corner of the full frame at 1:1 scale. The two right hand panels show the results after dark calibration with the scaled dark. It cleaned up most of the slightly warm pixels and removed the pattern structure well enough to turn almost unusable data into good data...
Not saying that dark scaling is the best way to do business with a CMOS chip - just pointing out that, if all else fails, manual scaling actually works OK and darks can usefully be scaled to different exposure times. Provided of course that the darks to be scaled are taken with the same gain/offset/temp/USB as the lights. Not a big deal, but hopefully of some use to CMOS owners. Thanks for looking. regards Ray