Upper L/H: Line Profile of Individual Sub image
Lower L/H: Individual Sub image
Upper R/H: Line Profile of Calibrated image
Lower R/H: Calibrated image
Note how the Individual Sub image curve abruptly drops at around 27k ADU to about 11k and 14k. This abrupt change is referred to as "shoulders".
From discussions with knowledgeable folks on the Cloudy Nights forum, I have learned that ......
" ....... If a line profile through your images shows these "shoulders" you can assume the optical cone is being restricted by an aperture stop, possibly the filters. If the curve is smooth, even if it drops off dramatically in the corners, the light falloff is most likely a natural consequence of the telescope optics.
As a quick test, you could remove one filter and shoot through the empty filter position. If the vignetting changes (or not) you'll know if it is caused by the filter."
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6011701/page/4/view/collapsed/sb/5/o/all/fpart/2/vc/1#6020892
Note the strange ADU pattern of the Calibrated Sub curve. It starts at about 32+k on the left, drops to the 29k value and then falls to the 26k value on the right. You can also see that the stretched version of the Calibrated sub shows over bright corners on the left side and over dark corners on the right. Needless to say, the flats did not (could not) do their job.
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