Flats causing but blue patch in the middle

You need to supply the raw files, not the calibrated stack.

Correctable it is:

📷 image.png

Not the whole set, just a meaningful set, say 1 hours worth of data with the ancillary dark/bias/flat.

As for the correction the following has been applied:

1. cropping out the borders

2. ABE (linear only)

3. MSGC

4. Graxpert (process, not script), 2x

Craig Dixon · Jul 23, 2026, 07:08 PM

Thanks, I appreciate you taking the time. here’s one night or lights as well as calibration files. I’ve re-done the flats and dark flats many times using different capture methods but get the same result.

https://drive.google.com/drive/folders/1PbdhzaDQ2jpOoBUqdZGpHO0j5l7fKFOG?usp=sharing

I gave it a go and my take is that you got something wrong going on in either the filter or in the refractor, flats shouldn’t look like that. Also, you are slightly out of focus. Can you try out some other filter to see if the problem persists?

📷 image.png

andrea tasselli · Jul 23, 2026 at 11:45 PM

Craig Dixon · Jul 23, 2026, 07:08 PM

Thanks, I appreciate you taking the time. here’s one night or lights as well as calibration files. I’ve re-done the flats and dark flats many times using different capture methods but get the same result.

https://drive.google.com/drive/folders/1PbdhzaDQ2jpOoBUqdZGpHO0j5l7fKFOG?usp=sharing

I gave it a go and my take is that you got something wrong going on in either the filter or in the refractor, flats shouldn’t look like that. Also, you are slightly out of focus. Can you try out some other filter to see if the problem persists?

📷 image.png

I agree, the flats look terrible but this bright patch is present in the light frames too. Here’s a screenshot of the master light from WBPP without using flats.

📷 Screenshot 2026-07-24 at 09.30.50.pngI’ve taken some flats with no filter and am stacking the data now but they look very similar. Interestingly though, I also have some data with a DNB filter and these calibrated perfectly.

this is the first project I’ve started with the scope in this configuration (It usually has a mono camera and different filters/wheel on). I haven’t yet taken the system to bits to investigate as if I do so, I can’t then re-do any flats.

Stellar Nomads · Jul 24, 2026 at 12:51 AM

was there dew on the scope? during imaging or flat taking? In my experience, that’s what would cause the center to bright up.

I’m certain there was no dew either when taking flats or lights.

andrea tasselli · Jul 24, 2026 at 10:29 AM

I don’t think the issue is with the flat. It could be the filter but it’s a bit extreme. Doesn’t look like a reflection either. But the bottom line is that the image can be salvaged so all is not lost. It would be nice to see if a more restrictive L filter (like L1) would solve the issue. A cheap SVBony UVIR cut should do, I think.

I get what you’re saying. Here’s a screenshot of a single flat with no filter so the light patch is pretty much gone. I originally bought the Astronomic L3 as I had a SW 72ED (Doublet) at the time and was advised that the L3 would take care of some of the chromatic aberration produced by a doublet. The FF65 I’m using now is a quintuplet so the L3 is most likely overkill. I have a ZWO UV/IR that I could put in to try for the next time out but I’d still have thought that if I shot the lights and the flats through the L3 filter, the flats should still calibrate out the light patch.

I think you’re right though, the L3 filter, whether it’s unsuitable for this scope or it has something on it, is causing the issue. I think it’s time to break down the system and inspect as no amount of re-shooting flats is going to help now.

I have had a similar experience with the 533mc except my spot was red, I also couldn’t figure out what was going on, I changed my uvir cut filter from the filter draw and put it in the redcat’s filter holder so it was further away from the sensor and wrapped a black sock around the imaging train. This solved my problem but not sure if it was the filter (it was 5mm too close to the sensor due to bound up threads) or if it was light leaks. Hope you get it sorted as I know how frustrating it is.