Strange noise in stretched image

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cuivenion
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Strange noise in stretched image

Post by cuivenion » Sat Feb 03, 2018 9:00 pm

Hi, This image was taken back in November 2016 with Sharpcap Live stacking with a colour GPCAM v1. I did stretch very aggressively to bring out the horsehead. I've not done EEA for a while but I'm looking to get back to it soon, so I was wondering what was causing this noise pattern? I was using darks with the live stacking, is it mismatched temperature wise?
Horsehead.thumb.jpg.b400e49a7cca39d6159bc37fff8a8693.jpg
Horsehead.thumb.jpg.b400e49a7cca39d6159bc37fff8a8693.jpg (149.79 KiB) Viewed 513 times

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oopfan
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Re: Strange noise in stretched image

Post by oopfan » Sun Feb 04, 2018 12:33 am

That's called 'raining noise'. It is due to less-than-optimal guiding. (The direction of the streaks in your photo are in right ascension.)

Try turning up your black level.

cuivenion
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Re: Strange noise in stretched image

Post by cuivenion » Sun Feb 04, 2018 1:30 am

Hi, its an unguided image. But I imagine that would apply for normal sidereal tracking as well. Would dithering get rid of it?

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oopfan
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Re: Strange noise in stretched image

Post by oopfan » Sun Feb 04, 2018 2:45 am

Never tried dithering, my mount is incapable of it, but from what I understand dithering requires active guiding. The telescope toggles one pixel between exposures: expose-move left-expose-move right, etc. I think it is used to "spread" the noise over two pixels instead of one. Robin could give a better explanation than I.

I too have issues with raining noise. I thought that it was just something I had to live with until a friend asked me to process a stacked image in StarTools. I could see in the initial stretch that there was a lot of stacking artifacts due to suboptimal guiding similar to what I suffer from. I was pleasantly surprised with the results - no raining noise. I now know the reason why. My friend spends a lot of time scrutinizing the histogram. He makes sure that there is a sufficient gap on the left hand side of the histogram. The secret is the black level setting.

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admin
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Re: Strange noise in stretched image

Post by admin » Sun Feb 04, 2018 2:07 pm

Were you using dark subtraction? This is often at its worst when you are dark subtracting and the dark is not an exact match for the light frame (often due to temperature differences). You end up with over or under subtracted pixels in each sub frame and the mount drift makes this effect out of them.

cheers,

Robin

cuivenion
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Re: Strange noise in stretched image

Post by cuivenion » Sun Feb 04, 2018 4:25 pm

Hi, yes I was using dark subtraction. I might be better off dithering and using the sigma stacking option until I can afford a cooled camera.

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