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Gras FITs are 16 bit unstreched uncalibrated, raw out of the camera, just like the FITs out of any Astrocam. The signal level is relative to exposure time. I always use the log stretch function in Liberator with (calibrated and stacked) FITs, otherwise, as you have found, they are very dark and need lots of stretching in PS.
Stacking/calibration programs screen stretch whilst processing, so this is all not obvious with them. I may misunderstand what your tying to do, but I guess you are 1st useing a stacking/calibration program on the GAS FITs, which also can stretch before loading into PS if you wish.
You cant just load GRAS FITs into PS as is, they at least need calibration which cant be done in PS. Calibration files are avaiable on the GRAS FTP site for the camera you used.
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