Thanks for the quick response. The camera does have a fair few hot columns, but they calibrate out without an issue when I run the standard calibration process in Pixinsight (including Cosmetic Correction, which I assume does more or less what Remove Bad Pixels does). The camera is quite new, and lightly used. I bought it Jan 2020 and started imaging Nov 2020, always from my backyard. Serial number is: x19110001. As to the flats, I use an Alnitak flipflat, and take the flats long before sunrise (see photo of set up above), and there is no light source near the scope. I uploaded some raw flats to share a couple of posts back, if that helps, but when I look at them, I can't see anything that would cause the issue. They seem about as expected. I recognize that you don't weigh in on processing issues, but might you give me some guidance on what to look for to verify if this is a flat issue or not? I would be delighted to find out that I simply needed to adjust my flat technique, but the ripple is so consistent across so many different targets taken over a several month period, that I worry it might be something else. I am happy to do the leg work and report back, it is just that I have exhausted the list of things that i know to look for.