andyofastro

Thank you. I think I now know what has caused the issues. I only took flats on the second night/morning. Potentially the scope focuser may have sagged/moved/temp variation/list goes on. Therefore I think my theory, which I need to test, is that if I had taken Flats at the end of each night/morning/session and calibrated each session accordingly before stacking all data, that this issue will hopefully not re appear. I will test on the next successive clear nights. Thank you for your help, it has really assisted with clearing my mind and getting a workflow.

Thank you for your reply. I'll blink through the calibrated frames tonight to see what I can see. As a wider question about imaging with Newtonian's and when to take flats: I would like to gather imaging time over multiple nights, which may necessitate removing the rig from the garden between imaging sessions. I am therefore assuming that I should be taking flats before removing the scope between imaging sessions. I am also assuming that those flats should only be used to calibrate the images from the same night. Then, only after all frames from multiple nights are calibrated with their corresponding calibration frames (Darks, Flats, Dark Flats), should the calibrated frames be stacked together? Thanks.

I took the flats after the second night (scope was left rigged) but would have been de powered between each night. Should I be taking my flats immediately after each imaging session? Like darks with a DSLR? Thanks

Thanks for the reply. I took my flats after imaging. The scope Parked around 4am and the flats were taken mid morning. However the camera would have been warmed and powered down between this time. Also it was two nights imaging, should I be taking flats for each night, straight after imaging? Thanks

Any ideas what is causing the large ring in my images? It is a 200PDS with a 269C camera, Baader Coma Corrector, L Pro Filter. I don't believe it is light leaks, the primary is covered up, focuser is covered up with felt and taped up at all joins, massive dew/light shield is used to stop stray light as well and most importantly, it is not present on individual subs. I have retaken all calibration frames, including flats, short flats, long duration flats, Darks, Dark Flats etc. The only other report of this I can find is on astrobin: https://www.astrobin.com/.../help-please-some-circle-is.../ There was no resolution described. It wasn't a 200PDS it was a 200P but very similar. Thank you for any assistance you may be able to offer.