Old 14-12-2018, 10:15 AM
Russj (Russ)
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Image Backgroud Problem

Hello all,
I have a problem with streaky mottled backgrounds in my last set of data, the data is taken with,

ASI1600mm pro, ZWO filterwheel, Zwo LRGB filters thru an 8" newt

90x60sec for each channel @75 gain, images were dithered every 3rd frame. The frames were taken over 3 evenings.

The pattern varies from channel to channel sometimes its at 45 degress, on other channels is vertical or just generally mottled.
I've tried processing in BPP and manually with similar results.

Can anyone suggest what might be causing this and/or a processing fix in PI.

Regards Russ
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Old 14-12-2018, 10:56 AM
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RickS (Rick)
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Is the pattern something you've never seen before? Are you doing flat calibration? CMOS sensors tend to have more fixed pattern noise, so you do need to be careful about calibration and dither (did you check that the amount of dither looked adequate in the StarAlignment output?) Longer subs or higher gain might help too.

Careful noise reduction with one of the Multiscale processes (I like MultiscaleLinearTransformation) should help and will allow you to target some of the bigger scale noise. Use a mask that protects everything apart from the background and after NR you should be able to move the HT black point to the right without clipping. You can also use an "S" curve with CurvesTransformation to darken the background further.

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Old 14-12-2018, 04:43 PM
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Almost looks like fixed pattern noise
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Old 14-12-2018, 06:39 PM
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Atmos (Colin)
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I agree with Lewis, looks like a small amount of FPN. This camera with no dithering has a LOT of FPN. I’d suggest that you either need a larger dither movement or to either more often.
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Old 14-12-2018, 08:50 PM
glend (Glen)
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What temperature were you running at the sensor? I never see any FPN on my old version 1 ASI1600MM-C with everything done at -20 to-25C. Over stretching should be avoided.
Have you tried various gain settings? I would contact Sam at ZWO, as sometimes driver tweaks have been needed. There are a fair number of threads on the Net, Stargazets, CN, etc concerning this very issue. The ZWO forum suggests emailing Sam directly for assistance.
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Old 15-12-2018, 01:51 PM
Russj (Russ)
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Rick, I've not experienced this noise before, MLT wont even look at it, I'm a PI beginner so my depth of knowledge is pretty low, I've darkened the background to lessen the effect, doesn't look to bad now.

Glenn, I run the cam at -15, if the problem persists I will contact Sam, I've spent a lot of time on CN looking for a solution, often with CN I come away more confused than enlightened, (some of us old blokes confuse easily)

Once the never ending cloud moves from here I will reshoot at gain 75 then at 139 and see if the noise persists.

Thanks to everyone who contributed
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Old 15-12-2018, 04:18 PM
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codemonkey (Lee)
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It's been a long time, but I'm pretty sure that looks like what I saw when I needed to change the desiccant tablets on my 1600
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