Bluish patch appearing in stretched images despite unchanged optical setup

have you checked the various surfaces in the image train for any polutants?

I had a similar issue on the window of my DSO camera where the temp & humidity interacted with cleaning chemical residue … created a foggy halo…

good luck

p.s does for camera have a dew heater??

My guess would be a flat issue, seeing I have been banging my head against them for two weeks on my last target using my 2400.

Tom

i’ll retake flats and make sure that my setup don’t have any leaks and check the sensor just in case

also same setup 2800mm + dual band filter 3nm + zwo 2400mc 600sec bortle9 same flat panel, same adu 32k (Took it on November)

Jorge Roque · Dec 22, 2025 at 01:17 AM

During the last corrector cleaning,which I did exactly a month ago I applied alcohol with cotton and unintentionally used circular motions in the central area rather than straight center-to-edge strokes. I believe this may have introduced a thin residue or scattering layer on the corrector surface.I’m going to re-clean the corrector tomorrow using straight center-to-edge strokes. I’m 100% confident this is the issue and not related to flats

Well, there it is! Great job figuring it out.

Jorge Roque · Dec 22, 2025, 01:17 AM

During the last corrector cleaning,which I did exactly a month ago I applied alcohol with cotton and unintentionally used circular motions in the central area rather than straight center-to-edge strokes. I believe this may have introduced a thin residue or scattering layer on the corrector surface.I’m going to re-clean the corrector tomorrow using straight center-to-edge strokes. I’m 100% confident this is the issue and not related to flats

Try mixing some Ether, up to max. 20%, into your alcohol but it should be Isopropilic alcohol at 99%. Normal pharmacy alcohol does not evaporate that good but Ether helps it to evaporate much faster without leaving any residue.

thanks! tomorrow first thing in the morning I’ll go to CVS to buy 99% isopropyl alcohol, distilled water, and optical wipes.

Did you consider ice on the camera lens?

I had an effect like your when on enviromente temperature like +5°C or more i set -10°C.

On my ASI294MC some ice born on the glass.

My new rule is when the temperature is more 5°C i set the cooler on 3°C, if the temperature is less than 3°C i set -10°C or less.

e.ferriani · Dec 22, 2025, 06:33 AM

Did you consider ice on the camera lens?

I had an effect like your when on enviromente temperature like +5°C or more i set -10°C.

On my ASI294MC some ice born on the glass.

My new rule is when the temperature is more 5°C i set the cooler on 3°C, if the temperature is less than 3°C i set -10°C or less.

But I’m in Miami, and nighttime temperatures are usually around 21 °C. I already ordered a full-frame sensor cleaning kit just in case for my ZWO 2400 sensor

SOLVED
After cleaning the corrector plate again, the problem was still present. At that point, I focused on the camera instead. I opened it and performed a deep cleaning of the sensor area, including the desiccant tablets (microwave for 1min), the heatsink, and the fan, which all had a significant amount of dust buildup.

After cleaning the camera, I was able to confirm that during a linear stretch everything looked normal again, with no patterns visible.

1 hour F10 Monkey Nebula from Bortle 9 Linear Stretch and BLX

Tom Marsala · Dec 26, 2025, 07:37 PM

So then what would be your analysis? Dew building up on the sensor area? Good to know!

yes, that’s my conclusion as well. It was most likely moisture buildup in the sensor. When I opened the camera, I noticed that one of the four desiccant tablets was partially loose, and when I regenerated them in the microwave they released a significant amount of moisture