Circular gradients in broadband images

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!"

andrea tasselli:
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.

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: 

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

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

Hi Kurt,

Thanks for the link to the tool; very useful!

Regards, Johan

David, I have seen similar and I would guess you’re using a cassegrain.

The cause is diffuse stray light, from either or both of two mechanisms.

a) stray skylight which enters the OTA at an oblique angle can reach the focal plane. If you point the scope at the sky during daylight, take the camera off and look into the back. If you can see sky anywhere around the secondary mirror that isn’t blocked by baffles, it is this. It is a particular issue when imaging with a light-polluted sky, moonlight, but a bright star outside the field of view can cause it, too. One solution is to use a dewcap extending well in front of the OTA to block this stray light.

Regarding this, note the full circle illuminated at the image plane is always larger than the camera sensor so yes there is always stray light around the sensor.

b) diffuse reflections  of a bright object in the field of view, off the insides  of parts inside the scope, particularly a central baffle, the extension tubes or screw threads along the imaging train, and even off the faceplate of the camera sensor. NGC5128 is bright enough to do this, ditto bright stars, and while metal parts are anodized black they’re still shiny at grazing angles. In my case it was the outside of the central baffle causing it, and I have just painted it with Black 3.0 which fixed it. So look carefully into the back of the scope while it’s pointed at the sky, any anything shiny should be painted black or possibly flocked. Other examples are secondary mirror cells and baffles, the machined insude surface of the corrector cell, th interior of a focuser, even the threads on the rings supporting the camera.

Lastly there is another solution - after imaging the target, slew to a nearby patch of sky with no big extended DSO or galaxies -just stars - and take 1 sub with exactly the same settings you used on the target - gain, exposure, filter etc. In your workflow, use this frame to make a “fake flat” by using the Inpaint tool to remove largest and brightest objects, then applying a median blur to obliterate all the stars - and subtract this flat from the stacked image. This will remove the gradient. There is a video by James Ritson showing how to do this using Affinity Photo, Photoshop can do similar.

I now use this “fake flat” method routinely in any case as part of my workflow.

I've seen something similar when shooting with mounted filters. I couldn't get flats that modeled the effect very well and always had these circular gradients. 

At the time I was using 1.25" mounted filters with asi1600.