Artifacts in stacked images with C9.25 XLT and reducer setup

Thats over correcting flats i reckon… that would be my first check. Try getting some flats with a lower average adu counts and some with a slightly higher average adu counts and re-calibrate and stack the data.

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HR_Maurer · May 18, 2026, 04:54 PM

im not sure if i can add some valuable thoughts. Shutter speed isnt the problem i guess, since you’re already aware of its limitations.
However, if i look at your example image, there are these dark circular patterns on the left. I also see bright edges at the right, which might be circular too. It could be due to internal reflections, and some sort of tilt.
However, something else just came into my mind: it could also be the result of “etaloning”, i.e. a nonuniform coating on the CCD or some other element causing interference patterns. In that case, these patterns should be reproducible per filter, and also appear on other telescopes. I’m not sure if i experienced that on my 383L back in the days, but i had it on an IMX183 some years ago.

Thanks for the response. I take it into consideration. I still think a slight mirror shift can be the cause (the pattern did not appear when shooting through r, g and b and the scope was already pointing quite close to vertical, and I take flats with scope pointing (almost) vertically. If there is an issue with coating, I guess there is not much I can do. Regards Ørnulf

It does look like a flat calibration issue to me. I don’t think it has anything to do with reducer spacing but more to do with something changing between your light frames and your flat frames. I suppose mirror flop could cause that but again, I don’t think so. Here’s a simple test you can do. With the scope pointing near the zenith, take a series of subs, maybe 30 min. worth. Then, without changing a thing make a set of flats. To eliminate issue with your flat panel. power down the mount with the OTA still looking up, freezing it’s attitude. Wait for just before sunrise and make a set of sky flats. If that calibrates properly. Just remember that exactly the same means just that. Dew shields, focus position, filter, camera offset and gain, everything needs to be the same. I even bring up the cooling just to be sure. The only thing that changes is the exposure time needed to get the histogram peak between 35% and 50%.

Jan Erik Vallestad · May 18, 2026, 11:28 PM

oernulfs · May 18, 2026, 06:23 PM

Thanks for your answer. I know you have adviced to this in another forum…. No I have not tried it yet. I like the FOV the set up gives, so I will not get rid of the reducer quite yet. In that other forum you mentioned something about “resampling” an image taken at f10. I must admit that I did not understand that information…

What do you think about using my zwo asi 6200mm at f10 on the c9.25? (also something I have not ried yet)
Regards Ørnulf

I understand with regards to the FOV, but if it is internal reflections from the reducer there’s not much to do. Such reflections will ruin every image you take. It’s well worth it to do a test, if only to rule it out.

Resampling is done in post where you change the pixel dimensions, effectively enlarging or (as in your case) shrinking them. This is what we refer to as software binning. If your mount is capable enough to image within the required image scale/RMS it’s not an issue - but if not a downsample will be beneficial to avoid oversampling and increase SNR. In short; You downscale your image, combining four pixels to one.

It depends on your setup. I think the imaging circle is large enough to support full frame without the reducer, the reducer reduces the imaging circle (obviously). Then there’s the matter of guiding. I’m not sure if an OAG would suit full frame as it needs to protrude quite a bit to get enough light, just a thought as I don’t know for sure. So I reckon it depends.

I think both my Edge 8HD and the 9.25 shares the same imaging circle natively (42mm), and with APS-C I have to keep the OAG on the outside of the sensor area, closer to the edge of the tube which reduces the amount of light the guide sensor gets. Getting enough light to the guide sensor can be a pain depending on where you want to image. If the OAG casts a shadow on the sensor, flats might be able to fix a bit of it - but not if it’s too much of an intrusion.

I’m sure people are imaging with full frame though. Cropping is always an option alongside downsampling (if needed).

Edit: the number of pixels combined or expanded is of course dependant on how much you rescale the image, i usually do bin2 go keep within the necessary image scale. But with an EQ8 you might not need to.

Thank you! If I (we) only had more time under the night sky to experiment. When fall comes I will try without reducer, and maybe I will try with the zwo. I will also investigate the resampling in pi.

Marcin Cikała · May 19, 2026, 10:05 AM

Hi.

This is definitely a problem with the reducer. Despite the declared large imaging disc covering the APS-C size detector it is impossible to shoot using KAF-8300 or almost equivalent IMX294 sensors. A ring-shaped highlight will always appear in the corners.

Take some photos without the corrector. Do not worry too much about the star shape or ideal focus. Reduce, combine and see what happens.

Best regards. m.

Thank you. It seems that the majority of the responders points at the reducer. I will try it., even though it narrows down (litterarly) the kind of objects I can image with this telescope and camera. I will look into the posibility to use my zwo asi 6200 in f10. Maybe that will satisfy my fov-wish.