I have been shooting a new PN discovery over the last month and that has caused me to look much more closely at my calibration and background values. As you may know, PI doesn't allow negative values so any post-calibration negatives are set to zero. My prior calibration program, CCDStack, allowed negatives so I never noticed this problem.
With my very highly stretched NB masters I was seeing a blotchy, uneven background and adding offset did not solve the issue. It turned out that the dark master was properly reducing my pixels to about zero, but random variation in the background pixel values of the individual subs meant that subtraction often produced a negative value. Normally that random variation would average out since it is both up and down but PI chops it off at zero. This was affecting nearly half of my background pixel values.
Just as I was considering manually adding maybe 500 counts to my light subs before calibration, the new WPPB 2.1 script was released with just this option! And it works like charm. I'm adding 100 counts to my 1x1 and 500 to my 2x2 subs. Further refinement to follow.
Rather than detail how this works here, just view this great video Adam Block has put up for free: https://www.youtube.com/watch?v=M4Bie7NOXbg
I think with NB sub calibration, and certainly with my QHY600M, this added pedestal will be used on every sub.
With my very highly stretched NB masters I was seeing a blotchy, uneven background and adding offset did not solve the issue. It turned out that the dark master was properly reducing my pixels to about zero, but random variation in the background pixel values of the individual subs meant that subtraction often produced a negative value. Normally that random variation would average out since it is both up and down but PI chops it off at zero. This was affecting nearly half of my background pixel values.
Just as I was considering manually adding maybe 500 counts to my light subs before calibration, the new WPPB 2.1 script was released with just this option! And it works like charm. I'm adding 100 counts to my 1x1 and 500 to my 2x2 subs. Further refinement to follow.
Rather than detail how this works here, just view this great video Adam Block has put up for free: https://www.youtube.com/watch?v=M4Bie7NOXbg
I think with NB sub calibration, and certainly with my QHY600M, this added pedestal will be used on every sub.