On various threads about FWHM, I have seen comments to effect that FWHM in arc-sec is a normalized form that allows comparison among different systems. In particular, it made possible certain reasoning about 5" FWHM being "bad" and 2" FWHM being "good" etc. (Yes, I understand this all depends on the seeing.) With this in hand, I have learned that I average about 3.5" FWHM. This is with 1.61"/px image scale, and about 3"/px guide scale.
I recently changed from flattener to 0.79x reducer for some targets. I assumed that since I was doing FWHM in arc-sec, and my mount and guiding setup had not changed, I should expect the same FWHM. But I have enough nights now to know that it is not. I consistently average around 4.7 which is suspiciously close to my original scaled by the change in image scale. (The new image scale is 2.03"/px so not very undersampled.)
Am I reasoning about this correctly? Should I expect the same FWHM in this situation where guiding/mount hasn't changed?