Author Topic: Need help removing circular bands  (Read 553 times)

Offline KeithL

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Need help removing circular bands
« on: 2018 April 11 19:01:45 »
I've been using PI with good success for over a year, but am far from being an expert and still have a lot to learn. Most of my past processing involved images acquired by telephoto lenses on a Nikon D810A DSLR. Several months ago, I purchased a Stellarvue SVA 130TIS refractor and Astrophysics Mach1GTO mount, and have been using it with my Nikon D810A during some rare instances of clear skies this past winter. I have a .72x focal reducer/flattener, and also a plain flattener. In processing images acquired by both flatteners, I have noticed circular bands in the images that I cannot seem to get rid of completely. They are most pronounced when using the plain flattener. At first, I thought that I was not creating good flats, so I tried creating new flats by several different methods (blue sky with and without white acrylic panels, white display screen with and without white acrylic panels, t-shirts, etc.). None of these made any difference in the results. Attached are examples of what I am seeing after running BatchPreprocesing on images acquired with the Stellarvue flattener. Nothing has been done to these images other than an STF stretch. Can anyone give me some advice on what I am doing wrong or how to get rid of these artifacts?

Thanks,
Keith

Offline sharkmelley

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Re: Need help removing circular bands
« Reply #1 on: 2018 April 12 12:49:34 »
With some DSLR cameras, colour variations sometimes happen in the shadow areas of an image.  The Nikon D810A is one of those cameras but it's not as bad as Sony cameras.

Where is the peak of your back-of-camera histogram for both the lights and the flats?  For the lights it needs to be at least one quarter of the way from the left hand side and preferably further for the flats:
https://www.cloudynights.com/topic/557931-concentric-coloured-rings-caused-by-flats/?p=7563882

Mark
Takahashi Epsilon 180ED
H-alpha modified Sony A7S
http://www.markshelley.co.uk/Astronomy/

Offline KeithL

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Re: Need help removing circular bands
« Reply #2 on: 2018 April 12 13:34:53 »
Hi Mark,

The histogram peak on the images were one quarter from the left edge, and the flats were at one half. I've never had this happen when using telephoto camera lenses on the D810A, so I wondered if it has something to do with the optics of this scope/reducer/flattener, but I wouldn't expect this from such a high quality scope. You commented in the DPReview Astrophotography forum bringing up the concentric banding on one of my images (https://www.dpreview.com/forums/post/60858052). That's the first time I noticed the problem, so I made sure that the exposure on this one was at least a quarter past the left edge on the histogram.

Thanks for the link to the Cloudy Nights post. Next time, I will use a higher ISO value and push the histogram toward the one-third mark and see what I get.

Thanks,
Keith
« Last Edit: 2018 April 12 13:49:58 by KeithL »

Offline sharkmelley

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Re: Need help removing circular bands
« Reply #3 on: 2018 April 13 10:58:20 »
If you are already exposing with the histogram one quarter of the way across then maybe something else is going on to cause these rings.  It's difficult to be sure.

Mark
Takahashi Epsilon 180ED
H-alpha modified Sony A7S
http://www.markshelley.co.uk/Astronomy/