Need help with Flat calibration - Forums

Topic starter

10 hours ago

Hi guys,

I’m in a big problem for two months and couldn’t find a solution. In my remote setup of Skywatcher-250 Quattro, I’m consisting getting dust donuts in low height areas, maybe light polluted sky area.

I know that easiest way is clean the filter, and that’s what I did first.. Even after the filters are cleaned, I’m still getting the donuts. Also, in my minds the donuts are from movement of dust. However, I have 2 sets of flats, on Feb.06 and Feb.14, and image was on Feb.07. If the donut keep moving, I’ll get different donut sizes, as in Feb.06/Feb.07/Feb.14, donuts are all in different places, and donut appear in 2 places: 06/07 error and 07/14 error. In fact, I used two sets separately calibrate and integrate, final image look identical. That’s after cleaning the filter.

Integration with Feb.06 Sky FlatsIntegration with Feb.14 Sky FlatsThese 2 pictures are after DBE and standard HistogramTransform, but I just want to illustrate if you download them and switch, the donuts sizes are identical. Also, I find it’s not a standard circle, looks like with a smaller circle inside. Is it some kind of reflection?

I’m adding the files here: https://drive.google.com/drive/folders/1g3xWZgNg5Ql8UdD8Ptmv3ejSig81N3hF?usp=sharing

Thanks in adavence,

Yingtian

YingtianZZZ

Topic starter

10 hours ago

📷 image.pngThis is the CCD inspector result of distance of dust, it’s a Skywatcher-250 Quattro, with QHY268M, so with 24mm distance, it should be on the filter. Only Red filter has this issue but LGB are not having the problem.

Tony Gondola

Frequent contributor

10 hours ago

IMO this looks like dust that moved slightly between the flat exposure and lights exposures, thus the light and dark donut pattern. The only real cure is to take your flats as close in time to the imaging session as possible. Ideally, right before or right after. Between filter wheel movement and scope orientation changes, dust can move. The best cure is to take it all apart and clean to a very high standard and don’t try to depend on flat libraries. Like donuts, they are best when fresh.

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YingtianZZZ

Topic starter

10 hours ago

Tony Gondola · Feb 17, 2026, 12:24 AM
IMO this looks like dust that moved slightly between the flat exposure and lights exposures, thus the light and dark donut pattern. The only real cure is to take your flats as close in time to the imaging session as possible. Ideally, right before or right after. Between filter wheel movement and scope orientation changes, dust can move. The best cure is to take it all apart and clean to a very high standard and don’t try to depend on flat libraries. Like donuts, they are best when fresh.

I’ll do a test of 02/06 integration, but as 02/06 and 02/14 flat clibration result look identical, if it’s a dust, it come back and forth from same place on 02/06 to another same place on 02/07, then back to same place on 02/14, for me this sounds very unlikely to happen. The dual circle pattern make me think of the secondary mirror, but I don’t know if it’s really some internal reflection

Brian Puhl

IOTD Staff

9 hours ago

Filter wheel may not be centering up properly. I had this issue recently with my QHY wheel, updated the firmware and fixed it. If you have a QHY, they publish the tool on their website, basically gets rid of unidirectional.

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YingtianZZZ

Topic starter

9 hours ago

Brian Puhl · Feb 17, 2026, 01:05 AM
Filter wheel may not be centering up properly. I had this issue recently with my QHY wheel, updated the firmware and fixed it. If you have a QHY, they publish the tool on their website, basically gets rid of unidirectional.

This sounds very likely to be a problem. Yes it’s paried with a 2’’x7 QHY filter wheel, and everytime I’m imaging flat frame with order LRGB as the filter number increasing, but in imaging sets I’m doing L-B-R-G, which means there is a B-R rotate and that may be a problem. I’ll first figure out this point.

Brian Puhl

IOTD Staff

8 hours ago

Same exact wheel i’m running as well. Mine worked for probably 2 years before this issue popped up. I had seen it and fixed it before on another system so it was a fairly easy diagnosis.