130-PDS help! Vignete pattern changes with focus point!

[pipnina]

I've been thinking my secondary collimation was wrong for ages, finally spent an afternoon tweaking it and it seems to look good as best as I can tell.

I went to test it by putting my camera back on, putting the cloth over it and pointing it at the sky out of the window to find the vignetting had maybe improved. But I was still seeing it appear brighter on one side than the other (left in landscape, right when I turned the camera 180 degrees!)

I also noticed it was changing based on focus position, weirdly, with the focus wound in it seemed to be worst, while it got better as focus was wound out, FITS files:

Has anyone else noticed this with the 130-PDS? Is this a kind of focuser sag or something to do with the size of the primary changing (from perspective of camera) as focus shifts, or my collimation not being right? I have noticed my Baader MPCC MK3 won't sit completely snugly in the focused (it always has just the smallest amount of play, whatever I do)

I'm a little confused myself as to the exact cause.

Cheers

HALF-WAY.fits ALL-OUT.fits ALL-IN.fits