Help with weird circular pattern in stacked image

16 Mar, 2018 22:42
Hi all,
have stacked 100 lights (30s @ 800ISO) with 5 darks  (30s @ 800ISO) and 5 flats  (1/400s @ 800ISO) using dss…The images are obtained  with a canon 600d with a sigma tele lens. I get this circular pattern that I do not know where it comes from. Weirdly enough I get the pattern both using the lens and on prime focus to the telescope (on other images), so I guess it's either the sensor of the camera or the result of the stacking/dark-subtraction process.
Any thought is appreciated!

17 Mar, 2018 00:39
If I were you, I would up my calibration frames to 20 Darks, 20 Flats and maybe 50 to 100 bias.
Make sure those flats are at mid range on your histogram and somewhat consistent.  That should calibrate most any issue out.  I don't know anything about DSS, but have heard some people say they like it.
17 Mar, 2018 01:49
It is because of your sensor. You have to use dithering to get rid of this effect. That is, slightly change position of your target in each frame.
Edited 17 Mar, 2018 01:51
17 Mar, 2018 08:35
Thanks for the answers so far!
As for dithering…the mount of cpc 800 is alt-az so I have field rotation, i.e. the image of the target changes position with respect to the sensor from a frame to another…isn't that some sort of natural dithering?…and perhaps the only benefit of having field rotation?!
17 Mar, 2018 08:41
No, it is totally different from dithering! Dithering is supposed to be random. And amplitude of movements has to be larger, maybe at least 10 pixels from frame to frame (it is more like more empirical thing, try different amplitudes, in general larger amplitude will work better, but you lose more and more of you FOV). Field rotation is slow and deterministic and for sure is the cause of your problem.
Edited 17 Mar, 2018 08:44
17 Mar, 2018 11:30
I would agree with it most likely being due to field rotation due to the Alt Az mount.
I have never actually seen this before, but seems the most logical.

17 Mar, 2018 16:37
Yup, looks like walking noise due to field rotation. Dithering should still get rid of it.
Edited 17 Mar, 2018 18:23
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