Thread: Lines in noise
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Old 07-12-2019, 05:18 PM
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DiscoDuck (Paul)
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Join Date: Apr 2014
Location: Adelaide
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Lines in noise

Hi,
I'm after some ideas on what may be causing the following. I put up an image of NGC 1097 recently and Bart noticed some streaks across the background. I've been trying to investigate this and am a bit perplexed.

The lines are shown in the attached image - stretched drastically. Next to it is a similarly stretched (same ADU difference from black to white) dark image. The dark doesn't show that pattern (nor does a flat). You do see lines in the amp glow area of the dark (other attachment) when extremely stretched. though they are at a different angle and probably a separate thing I'd think.

I stress that this is not a huge problem. The stretch I've used here has a 2 electron difference between black and white, i.e. the outer arms of the galaxy where you can see this background are themselves only about 1 electron above the background (in a 60 second image).

Suspiciously the lines are in the N-S direction, so I don't think it's sensor related but more guiding/tracking. Dithering was used - SGP and PHD2 (scale factor 1 in PHD and I think maybe 2 pixel or so dither chosen in SGP, though it could have been the next one up). And looking at the logs and skipping through with blink dither seems to be doing a good job of moving in BOTH RA and dec - though perhaps I noticed in watching the alignment occurring in PI a slight tendency to movements of the images along that diagonal axis if anything.

Could dither not be being that random?

Could it be a backlash issue? (Though I naively feel that this would show lines in the EW direction as it would tend to fail to move NS then) (the mount has about 400ms backlash).

I think I can reduce the impact of this by taking longer exposures (i.e. much more than my 45-60 second sky limited exposure time) and dithering more drastically, as I think it's to do with fixed pattern noise. Or perhaps I should not dither at all (probably not a good idea) as the lines are more distracting to me than just a generally reasonably uniformly noisy background.

Allan (alpal) and I have been having some really good discussions in trying to understand some issues in the image. But any advice as to what exactly is causing these lines would be much appreciated.
Attached Thumbnails
Click for full-size image (Dark vs Stack small.jpg)
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Click for full-size image (Dark lines small.JPG)
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