A slight illness stopped me for a day or two. Onward...
well i was asking because thing can change due to the interpolation happening to a registered frame. i guess i wouldn't expect the FHWMs or other properties to differ wildly after only calibration. but i have never run the experiment you have done.
Rob, I still haven't compared registered subs statistics against their non-registered stats, since the PI subframe selector script ( and now the new process) wants calibrated, unregistered subs to work on. I will get around to comparing some calibrated registered subs' statistics to their raw statistics in the next day or two, as I feel better. BTW, the initial observation was due strictly to serendipity, I accidentally used raw versions of the subs I was interested in when using SFS, and just happened to notice the stats were better than the cal'ed sub stats. Not really an "experiment."
This seems pretty odd. Calibration with darks and flats should not actually be changing the small scale nature of the signal. A change in eccentricity would be pretty odd after simply subtracting out a dark. I wouldn't expect FWHM to change much either, however it may depend on how the signal floor is identified, and calibration may have an effect on that.
A change in SNR weight may occur if you are clipping data. I would make sure you do not have very many pixels clipped to black after calibration, and if you do, add an output pedestal to prevent that.
Jon, I also find it odd, and agree with everything you stated (I'm NOWHERE near as sharp as you on this stuff, but I try to keep up). A while back, I thought I saw that I was clipping some pixels to black, so I studied up a bit, and started adding an output pedestal. In the Output Files section of the IC process, I have 1000 entered in the Output Ped (DN) box, and it appears that the ped is added to all my subs.
I wonder if it has something to do with the flat frames correcting for the vignetting. It is possible that once the vignetting has been corrected for it is picking up more stars in the outer parts of the image.
Your best way of doing a comparison would be to compare the same image before and after calibration. If they stars aren't changing then it is how Nebulosity is checking FWHM rather than the images becoming worse.
Atmos, I would have to think that what you stated accounts for some of the (negative) change in stats I've observed between raw and cal'ed subs. However, I've used Neb4.1 to check the same individual stars (by placing the pointer on a single star, and reading its HFR in the Pixel Stats window) on raw and cal'ed copies of the same sub, and seen the reported HFR go up after calibration. The Subframe Selector gives an "average" of the image star fwhms, if I'm not mistaken. My short investigation has so far revolved around checking single subs, before and after calibration, with PI SFS (for image "averages"), and with Neb4.1 (for individual stars) . In any of the cases where the stats got worse after cal, the images always appeared the same or better post-cal than pre-cal; of course, my eyes aren't what they were even ten years ago.
Edited by twidget, 09 December 2018 - 08:27 AM.