I purchased an ASI1600MM-P about a year ago, along with the 7-position EFW, Baader narrowband filters and Astrodon LRBG filters. I noticed some odd artifacting in the OIII filter right away, but a post on Cloudy Nights suggested that it was due to imperfections in the filters and it could be processed out with flats. I had (moderate) success processing out the problem in the beginning, but it seems to be getting worse and more evident in other filters.
Here is what I have tried so far to diagnose the problem. If I rotate the camera, filter, FF/FR, or telescope, the artifact stays in the same position. I have tried different FF/FRs and telescope combinations, but no matter how I configure it, it stays the same. I also have an ASI174MM (not mini) that I have attached to the same imaging train (filters, FF/FR, telescope) and I could not get the artifact to replicate. The spacing was considerably different since the attachment for the ASI174MM is not the same, so I experimented with different spacing for the ASI1600, but no luck. I have inspected the glass in front of the sensor to the best of my ability and can see no residue or imperfections. All other glass in the imaging train shows no abnormalities either.
The problem is most evident in the OIII and green filters, but it can also be seen in other filters to a much lesser extent. I have attached (via a Dropbox link below) a master flat from the OIII taken from over a year ago with a WO Z73. Also attached is a green master flat taken last month with an AT130mm refractor, and also a master light of 60, 60sec exposures (1 hour). All three have a basic stretch in PI using the STF, nothing else. You can see how both are nearly identical in shape and size, but more clearly defined in the OIII.
Since the artifact does not rotate with anything else, and since it was not evident with the ASI174, I was thinking it must have something to do with the ASI1600. Perhaps some imperfections in the glass in front of the senor? What else can I do to determine what's causing the issue? Thanks for help. Toby
https://www.dropbox.com/sh/u3mk5yg10q8aad1/AAA8TkeiwrjeheGo3A8f3r1da?dl=0
