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Hi All, Hoping someone might have an insight for me. In the attached images there is a really weird circular gradient arount the subject. I was imaging in a Bortle 5 area, LRGB using an ASI 1600MM and ZWO filter wheel. I have tried to process the gradients using PixInsight Gradient Correction, ABE and DBE with no luck whatsoever. In desperation, even tried Gradient xTerminator in Photoshop, again with no luck. The attached images are the masterlights in LRG and B, stacked using WBPP with a STF and saved in JPEG. No other processing. The masterlights were processed with flats, darks and bias frames. I have wondered whether it is likely to be an OTA issue, but I imaged Carina the previous night in narrowband (HSO) with the same setup and did not experience the same issue. Would really appreciate any insights that you may have. Best, David     |
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It boils down to one single line: do not use WBPP! 9 out of 10times the issue is something got wrong in the WBPP many many fields to fill in. You may get away with not so good calibration in NB because, well, it is NB and the background is supposedly faint but you can't cheat with BB, especially with L. Repeat each operation on its own as exemplified below: Make sure that Master Dark contains bias and the Master Flat had the Master Bias or Master DarkFlat removed. |
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| I've encountered similar results with a handful of images after stacking. I don't believe it's an optical issue because, as you've already pointed out, it's not consistent and only appears in some some imaging runs. The problem might be related to the calibration steps. Try stretching some individual frames to see if the issue is present in all of them. You might observe gradients and uneven backgrounds. If you notice these 'doughnuts,' it could be related to the imaging train.Otherwise, consider trying a different stacking method or using alternative stacking software like Siril or ASTAP to see if there's any improvement. Additionally, using a background extraction tool could help equalize the background and potentially resolve this issue in most cases. Give that a try and see how it goes. Cheers!" |
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| NB wouldn’t show the same broadband gradients. I suspect internal reflections from some aspect of the OTA. |
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andrea tasselli: I don't think it's WBPP. (There are just as many variables to fill in when doing it from scratch.) Case in point - I suspect my reducer is causing these rings, since they showed up after installing a moonlite litecrawler, and the reducer sits inside the device itself. From scratch:  WBPP:  |
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| Those aren't the same issues. You have internal reflection he has over-correcting flats. |
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Thank you to all who have responded. @andrea tasselli , I will give your suggestion a try, but I will say that I have always found WBBP has worked for me in the past with OSC and narrowband processing. I think this is the first time I have tried broadband LRBG, so anything is possible at this point. @Guillermo (Guy) Yanez , that sounds like good advice! I have not stretched any of the subs yet but will go and have a look very shortly - I am wondering if it is actually a problem with my flats as I have had some difficulty with those in the past with poor quality flats actually introducing artifacts into the processed image. And I do have APP so I can have a go at stacking there - it has often worked well for me in the past. @Eddie Pons , I am desperately hoping that it is not an OTA/focusser/reducer issue, but I will keep this in mind. Nice detail on the Needle by the way! Once more, thanks to all of you - will start experimenting tonight and will come back when/if I find the solution! Best, David |
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Hi David, I used to have circular gradients and found them hard to remove in PI, even with flats. I'm imaging in a Bortle 8-9 city area with lots of garden and street lights around. In my case the circular gradients were caused by stray light entering the internal baffle of my Sharpstar SCA260, where the baffle opening was a bit to wide. I fixed it by 3D printing a cap that fits on the baffle to tighten the opening by a few mm. Works like a charm! Not sure what scope you have but maybe this is of help. The cap;  Before cap;  After cap;  Regards, Johan |
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Hi @Johan Bakker , Thank you for this - I'll keep it in mind - I have not been able to image since I published this post (life and weather) but hope to get out once the clouds clear.... I have swapped out ny unmounted RGBL filters for mounted filters to see if they were causing the internal reflections. Fingers crossed. I am pretty certain it's an internal reflection that is causing the issue - does not happen when imaging OSC, which points to the problem being something related to the filter wheel/filters. I'm hoping the mounted filters are the fix. We'll see.... Best and thanks for your suggestion, David |
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Just my 2 cents: You could consider to see if there any dust motes in the imaging train, especially for you, Johan. There is a dust motes calculator which helped me to identify motes on my cameras sensor glas: https://astronomy.tools/calculators/dust_reflection_calculator Wit this this calculator I could eliminate the motes and delete any circular shadows as seen in Johanns images. |
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Hi Kurt, Thanks for the link to the tool; very useful! Regards, Johan |
