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
This Individual Sub image was shot through an open port on the EFW2 filter wheel. Consequently, there was more clearance for the light path than when the Luminance filter was used. Note how little the Individual Sub curve drops as a result of the "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 fairly normal ADU pattern of the Calibrated Sub curve. It starts at about 16k on the left, and then finishes just above 15k on the right. You can also see that the stretched version of the Calibrated sub shows none of the distinct vignetting that plagues the Luminance filter version. It appears that in this case, the flats did do their job. There is some gradient from left to right, which can be removed through processing.
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