So here's my camera set-up. H-a modded Canon 600D, with APS-C sensor. Rokinon 135mm f/2 lens, 77-49mm and then 49-48mm step-down ring, Optolong L-eNhance filter:
Annotation 2020-09-22 091451.jpg
Here's a 300-second light frame, uncalibrated, highly stretched in APP to show the vignetting:
Annotation 2020-09-22 091601.jpg
Here's a single flat frame, uncalibrated, highly stretched in APP:
Annotation 2020-09-22 091726.jpg
Here's the calibrated Master Flat, background neutralized and highly stretched in APP:
Annotation 2020-09-22 091842.jpg
And here's the calibrated light frame, calibrated with the Master Flat and Master Dark, highly stretched in APP:
Annotation 2020-09-22 092019.jpg
Edit - My guess is that this would be a problem on full frame. But it does appear to be effective with a crop sensor camera.
So I have not tried anything like this for astro, but I use nice wide fast lenses with wide apertures a lot for terrestrial sports shooting, and occasionally am shooting through openings in fences and protective barriers that are a bit smaller than the lens opening. What I've found is vignetting is not per se an issue, but that the occlusion will reduce overall contrast. In daylight it is worse if the occlusion has color or is lit, not something you have with stepdown rings of course. This is not like (say) a bad lens hood on a longer, slower telephoto, where you definitely see vignetting. F2, F1.4 - those apertures, with stuff near the glass, are just plain different.
I know zip about this in AP-land, but if it were me and I wanted to test the impact, I'd also take the filter off (just the filter) and shoot with and without the stepdowns for the same exposure, and see what FWHM and similar measures look like. Maybe there's no impact (like I said -- no color, dark, very round no shape -- maybe no impact except a slight light loss). But... maybe.
I am interested because I have an 85/1.4 I'd like to try, and it takes a 77mm if I recall.
I'm still waiting on a drop in filter (40.5mm) that is being made -- Astronomik will make filters to order, by the way. I didn't find it expensive (compared to similar sizes), though I also am still waiting for it so no comment on quality, etc. Though they seem to have a good reputation.
Edited by Linwood, 22 September 2020 - 10:50 AM.