discardedastro

I've flocked the inside of my tube now - took out the primary and very carefully cleaned it while I was at it. Once I had it all back together on the bench I assembled the camera and took some 60 second and 5 minute exposures. With the scope in a bright room there were similar-looking halos in the frames. Covering the focuser made little difference, but covering the rear of the scope by the primary with a dark coat resulted in perfect frames showing very no sign of any light. So it looks to me like I need to find a way to properly cover the rear of the tube to minimise the risk of light entering the tube and causing these problems again - either I can find an appropriate plate pre-made or get a friend to CNC some sheet aluminium. Thanks all for the help!

Right - thanks for all the tips. The scope is set up in my garden but still close enough to the house there are some potential sources of light. I've got the scope off the mount while I fit a focusing motor to the focuser (which has turned out to be Exciting, the Skywatcher focus motor mount does fit, but blocks up the parallelising adjustment screws, so had to expand some holes in that plate with some files and a drill), so I'll go around with a torch on the bench and see where there's potential for leakage. I'm guessing something as simple as a blackout cloth over the back of the primary would be enough to stop any leakage there, though I might be brave enough to pop the mirror cell off and flock around the interior near the primary. The focuser's the other potential entry point - I've now applied flocking around that area to try and minimise the impact of any leakage, but might also paint the (non-contact) external surfaces of the drawtube with some matte black enamel I've got lying around to mitigate bounced light there. Everything upstream of the focuser is sealed and I can take perfect dark frames with the lens cap on the EFW in a brightly lit room, so that lot's OK!

I should clarify though that the dark frame (second image in the post) was taken _with the telescope cap on_ - the really bright star on the right, do you mean in the last frame? If so, that's a dark frame taken with the lens cap on the EFW, rather than the telescope cap - I repeated dark frame capture with the camera cooled on the bench after I saw the first dark frames and the lighting issue, and the bright star in that frame is just the amp glow the ASI183 tends to suffer from. So I don't think it's a bright star causing issues - my thinking has mostly been local pollution, lights from near the scope etc. The Baader filters have a black mounting disc and I can't immediately see anyone reporting similar issues with them. I've got some flocking material I haven't yet fitted to the tube, so I might give that a shot - slightly terrified of removing the primary mirror cell to get at it all but hopefully it'll all go back together without too much issue! My suspicion is mostly that there might be light leaking in around the focuser's drawtube (on the outside) that's then reflecting off the secondary.

I've seen an interesting issue with my first few captures; the dark frames and light frames I took with the camera mounted on the scope featured this odd glow towards the middle/left of the frame. Conversely if I take a dark frame with a cap after the EFW with the camera on my desk, I get a perfect dark frame with the ASI183's usual amp glow. Is this likely to be a light leak, or something else altogether? Skywatcher 200PDS, ASI183MM-PRO, Mini ZWO EFW, all clamped up in the standard Skywatcher focuser.