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INDI Library v1.9.0 Released (23 Apr 2021)

Major INDI Library release v1.9.0 bring significant internal changes championed by @pawel-soja to modernize core INDI Library drivers and clients. New drivers for DeepSkyDad Flat Panel & Pegasus devices plus further improvements to PCM8 drivers.

Fixed pattern noise due to dithering

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Hi, I've been taking a few pictures lately and one thing that bothers me is a pattern (diagonal bands) that does show up in my final results. IMO it is due to sensor pixels that are more or less sensitive. When INDI dithers a certain amount of pixels and does so always in the same direction, something like this will show in your pictures (see attachment). Is my assumption correct that INDI will dither in one direction all the time? If so, dithering in a random direction should help avoid such patterns. As an alternative, dithering in a spiral pattern could also improve results. In case something like this has already been implemented, please let me know where to find it. Thanks a lot!
Last edit: 3 years 5 days ago by Herrhausen.
3 years 5 days ago #25919
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Hi Alfred,

"...Ekos will move the guide star in a random direction for the amount of pixels specified in the dither input box..."
Seems like it didn't dither at all. The pattern looks like drift due to imperfect polaralignment. I seen this in undithered captures I did myself, too.

Andreas
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3 years 4 days ago #25920

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OK, when Ekos uses random directions it must be misalignment. When I think about it... I did align pretty well but at the end of the night I noticed I had fogotten to tighten the azimuth screws. So it might have moved during the night. Bummer.

Thanks for your help!
3 years 4 days ago #25923

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You should not see this misalignment (showing in noiserain) at all, if dithering went well. Maybe dithering was just not wide enough. How much pixel did you dither and what is the ratio between guide scope focal length and main scope focal length?
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3 years 4 days ago #25925

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Main scope is 1200mm and guide scope is 1000mm. I never got to use the internal guider with my Alt-5. But this time I used a G-11 and internal guiding did perform very well. It reported dithering was performed successfully. I used 2,8 pixels every 1 frame.
3 years 4 days ago #25926

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This is the cause of your "undithered" looking capture. Asuming pixelsize is nearly identical on both cams you dithered your main cam 2.8*1000/1200=2.33 pixel.
This is near nothing. Hot pixel or "ununiform behaving" ones are neighboring themselfs in the stack, this makes the rain fall.
Dither at least 10-20Pix with this configuration to separate them for sure.
Andreas

edit its 2.8*1200/1000=3.3 but thats not enough either
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Last edit: 3 years 4 days ago by andreas.d.
3 years 4 days ago #25927

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Ok, will do that and report back. Thanks an awful lot!!!
3 years 4 days ago #25928

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I promised to report back... A few days ago I took another photo of M51. This time dithering was set to 9,7 pixels (10 pixels is the max in Ekos). The result is marvellous, noise is perfectly random IMO. Problem solved! Thanks a lot for your help, andreas.d!

P.S.: I had to abort the session after taking 13x3min exposures due to bad weather.
2 years 11 months ago #26059
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