Circular banding in images - any ideas?

[browndog]

I am hoping that someone may be able to explain what is happening here?
I have attached an image – which I have processed to exaggerate the circular banding that I have started to notice in my images.

This image was taken with an 8” SCT with a Meade f6.3 focal reducer / field flattener. It is a stack of 30 x 3min @ ISO400. Darks, flats and bias were applied.

I would like to know if there is something wrong with my setup? :shrug:
Can anyone help or offer suggestions?

[Dennis]

One candidate for this effect could be posterization? If the image is compressed too much, instead of a smooth, continuous gradation of tones from light to dark, you get a series of visible, discrete steps.

I have noticed this effect in daytime photos which include large areas of blue skies when the image has been converted from a 14 bit RAW file to an 8 bit jpg.

Cheers

Dennis

[AlexN]

Another candidate would be flat frame over-correction. If your flat frames is over-exposed, it can create effects like this.. I've had something fairly similar before myself when I was using a computer LCD screen for taking flats..

How are you capturing your flats?

[browndog]

On close examination the Master flat is also showing a similar banding effect - except lighter in the middle and darker at the edges.
The individual subs (15 stack) do not show this effect when opened in Photoshop.
All shots were taken with 350d (badder modified). I take flats using a simple "light-box". The histogram for each sub is around the 1/3 mark.



I am shooting RAW with my 350d (Modified) and then stacking with DeepSkyStacker.

The banding does not appear to be present in any indivdual sub - lights and flats. The individual flats show a nice smooth graduation of tones - lighter in the middle and darker at the extremities.

If I stack (in DSS) the same subs - but without any flats - I get similar circular banding, BUT lighter in the middle and darker on the edges.


Given that I can only see the circular banding after stacking - is there something wrong with DSS??? I am using the "recommended" settings.

[multiweb]

As previously pointed out this looks like a flat fielding gone wrong.

[browndog]

Last night I used the same setup - except changed ISO settings from 400 to 800.
Stacked using DSS on the same recommended settings.

No banding was present in the stacked image or Master flat?

So it seems that circular banding may be related to the ISO setting (specifically ISO400). So in future I will use ISO800.

[gregbradley]

Possibly a bit of dew forming on the corrector plate or some component inside the image train?

Greg.

[leon]

Yep It is definitely in the flat frames, once combined I to had this problem, and was unsure of what it was, but after healthy discussion on IIS it was sorted.

Not that i did much about it as it didn't really effect the final image.

Leon