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ZWO ASI 2600MC producing doughnut artifact in center of images

ZWO ASI2600MC Pro Artifacts Help Needed Troubleshooting
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Konrad Krebs avatar
Konrad Krebs avatar

Hello everyone,

I have had a ZWO ASI 2600MC for about a year now, and I have noticed that all the images I have taken with it since then have a strange artifact in them. Basically, it looks like a doughnut in the center of the image. This is particularly noticeable in stacked images that have been flat-corrected.

This varies slightly depending on the telescope used, so I suspect it is a reflection. However, I also use the same telescopes with an ASI 183 MC and an ASI 294 MC, and I don't have this problem there.

I've been experimenting for a while now and have found that both the lights and the flats contain the artifact, but it is much more noticeable in the flats. I therefore suspect that when creating the flats, more light is reflected strangely? And that this causes me to overcorrect for this artifact. Because all other errors (dust spots) are corrected correctly by the flats.

I tested flatbox as well as sky flats, no difference.

I'm now wondering why this problem only occurs with the ASI 2600MC, and even different filters don't really help.

The Sensor itself is not producing it, I checked with dark frames and also did flat without a scope attached, no issues with both.

Does anyone have any idea how I could solve this?

Setups I tested with:

Skywatcher 190 MN Pro + ZWO 2600MC Pro (no filter & UV/IR filter)

Skywatcher 150 PDS + Starizona Nexus 0.75 (UV/IR filter)

Omegon Pro APO AP 94/517 + Starizona Apex 0.65 (UV/IR filter & duo narrowband)

Following I attach flats (stretched to make it more obvious)

Flat from 190MN:

đź“· image.pngimage.png Flat from 150 PDS:

đź“· image.pngimage.pngThe flat from Omegon looks similar as from PDS. When I shift focus the general pattern is still there but is moving in size.

And finally an example of stacked image with over-corrected artifact:

đź“· image.pngimage.pngNo other editing done, just stacking and stretching.

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andrea tasselli avatar
andrea tasselli avatar
There is a light leak in the camera, possibly a reflection or a defective coating.
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Konrad Krebs avatar
Konrad Krebs avatar

Hi Andrea,

Thanks for the reply. I thought the same, but if I take dark frame with the camera being still attached to the telescope, I get a perfectly usable darkfram. There is no obvious light leak visible.

andrea tasselli avatar
andrea tasselli avatar
Light leak is just a terms for "added light not going through the optics", so a reflection qualifies as such. Obviously a dark won't reveal anything of the sort nor does experimenting with unfocused light. Check around for anything only even not absolutely opaque in and around the camera meaning anything that stays the same when switching telescopes.
Konrad Krebs avatar
Konrad Krebs avatar

I see, I will check again.

Do you think the UV/IR filter build in the camera could do that? You mentioned defective coating. A replacement window will cost about 85€.

andrea tasselli avatar
andrea tasselli avatar
It is possible but I would exhaust all the other possibilities first.
Ali Alhawas avatar
Ali Alhawas avatar

1-For the lights and flats that are affected, what was the exposure?

2-Is the issue appeared in any exposure?

3-Did you perform your tests in the same environment? lights! cars? home?

4-Have you ever get unaffected images?

5- Did you clean the camera sensor? If yes WHEN?

If I were you and the issue is one year old!! I will contact ZWO, maybe something wrong with the camera, sensor, Software or internal connection.

I will suggest you (For now) some steps,, (seem stupide :)

Change/ or use another:

1- cables, including the power adapter.

2- pc/ laptop.

3-acquisition app. NINA, Sharpcap..

4-open the images by another preview app. Windows pic., pixinsight, NINA

5-take some images by ASIStudio. and see!

They are all look unlogic, but they are easy steps to eliminate factors one by one till the root cause appeared.

Hope you will solve it soon!

Goodluck!

noon avatar
noon avatar

I’m inclined to think it is an internal reflection. These can come from filters, especially if they are cheap or uncoated. They can also come from uncoated or multicoated coma correctors, or if the clear glass window or protective cover over the sensor is not anti-reflection coated… or it could be a reflection off the sensor itself that bounces back forming an out-of-focus ghost image centered on the optical axis.

To diagnose:
Change the filter (if any): If the artifact disappears or changes shape, the filter is involved.

  1. Tilt the camera slightly: Internal reflections usually move in subtle ways when tilt is introduced.

  2. Shine a flashlight into the front of the scope and observe directly for ghost paths reflecting off the sensor.

  3. Try with no filter or no coma corrector: Simplify the optical train to isolate the component causing the reflection.

How to Fix or Mitigate:

  1. Use high-quality, AR-coated filters (double-sided anti-reflective coatings).

  2. If you figure out which surface is causing the reflection, carefully remove and clean that surface then replace and try again.

    I had something similar a while back that arose after I swapped filters in an EFW… turned out when I cleaned them I had left a very very slight haze to the filters that caught and magnified internal reflections. I did the above steps, traced the issue to the filters, removed and cleaned them very very carefully, and the problem resolved.

    Good luck

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Brian Puhl avatar
Brian Puhl avatar

light leak, reflection.

Since you say it followed all of your scopes, I would look at your imaging train particularly. Not uncommon for filter wheels and any adapters to have light leaks.

Put the lens/dust cap on the scope, pull up sharpcap while its dark (you could bring indoors too) and shine a flashlight around while doing fast exposures, watch the ADU/histogram. You will find it very quickly if it’s a light leak.

If it’s a reflection, shine flashlight down the tube looking towards your sensor, look with your eyes for shiny reflective edges.

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alpheratz06 avatar
alpheratz06 avatar

I see two possibilities:

Light leak , it has to be check as described above ( closed tube + flashlight)

Over compensation of flats : you must make sure that all the frames are shot with the same offset at camera level

I have been trapped a couple of times with old darks , shot with a different offset wrt to “fresh” light flat bias.

Offset change acts as a wrong bias and ruin the multiplicative flat correction.

Clear skies

SonnyE avatar
SonnyE avatar

I’ve had doughnut’s before but can’t recall exactly what got rid of them.

Not long ago when I added my Rotator to my imaging train I went to great pains getting my backspacing correct. That was actually a real PITA.

But while doing that, I threaded everything together from my focuser to my camera.

One was to eliminate any possibility of light leaks. But more important to me was to make everything rigidly affixed to the rest. No slip-fits, and no wobbles anywhere. Just ridgid from the 3.2” focuser back to the camera itself and everything in between.

But of the possibilities, it sounds like it is following the camera or imaging equipment train around. Make sure your back spacing is correct.

55mm (54.9mm) did not work for me. Mine is closer to 53.xx and I’m happy with my stars clear to the corners.

Good luck curing it.

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