Vignetting with 533?

[OK Apricot]

As I understand it, being a reasonably small sensor the 533 is quite forgiving in terms of field illumination, star elongation in corners etc, so why is it that I'm getting vignetting, or at least what looks to be vignetting? 

I'm just getting started with imaging with a newt so there's plenty to learn, just wondering what I'm seeing here and why. Using the SW 250 Quattro, SW Aplanatic F4 corrector. I checked collimation, while it's not perfect it's not far off, a smidgen of secondary rotation required if anything, the rest done by collimated laser. Seems to be calibrated out with flats. The screenshot was taken at 2200 so around 1.5hrs until astro darkness. 

Please could you help me understand and remedy? 

[michael8554]

M48 screwed connections ?

55mm from the Corrector to the sensor ?

SW say "28mm image height", which may mean the full vignetted circle.

Even so you'd think your 11mm x 11mm sensor would be in a good part of the circle.

Michael

[Elp]

I think you'll get vignetting regardless as the light at the edge has to travel further than the light in the centre, a spherical projection onto a flat image sensor also adds to this I think. Faster F ratio makes the light cone more steep from centre to edge also having an effect. If anything interacts with the edges, tubes, filters etc it can also contribute.

Unfortunately AP without flats isn't a thing yet unless you crop.

[OK Apricot]

@michael8554 threaded connections from the corrector backwards. The corrector itself is secured by the standard SW focuser thumb screws. 55mm backfocus is where I've started, though I'm fairly happy with the stars at this point so won't fiddle just yet. 

I guess I was surprised that I'm not even getting a fully illuminated 11mm image circle, seems kind of strange. I dread to think what each sub frame would look like from a larger sensor - you'd assume the scope practically unusable. I'll gradually chip away at blackening mirror edges, corrector edges maybe(?), mirror clip mask etc and see where it ends up.

[woldsman]

+! to taking flats. The other things to watch for with your scope are light leakage around the primary (black swim cap combats leakage, if it stretches enough) and

primary mirror misting.  Rigging up a  DIY 5v usb powered  fan will take care of the latter. eg

https://www.amazon.co.uk/Noctua-NF-A4x10-5V-PWM-Premium/dp/B07DXS86G7/ref=asc_df_B07DXS86G7/?tag=googshopuk-21&linkCode=df0&hvadid=205267456752&hvpos=&hvnetw=g&hvrand=2642092141730500132&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9046128&hvtargid=pla-700140532537&psc=1&mcid=34c4ad86cdb43a598ac7f89e54db59d0

Affix to a piece of plastic cut to the diameter of the base with indents for the collimation screws, then attach with some velcro/tape. The swim cap also helps hold the fan  it in place. 

These mods are something to do next time its cloudy.