Weird Histogram with Dither

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nexusjeep
Posts: 204
Joined: Thu Oct 18, 2018 3:01 pm
Location: Gloucestershire

Weird Histogram with Dither

Post by nexusjeep » Sat Sep 28, 2019 12:18 pm

Hi Robin,
I have not seen this before with the histogram when I have been imaging but last night was the first time I have used dithering as the new option for 1s means I do not lose 180s each time actual it usually took 12-16s last night per dither. However after each progressive dither I started to develop additional peaks to the left of the normal black level point which the auto stretch then starts to use as the base point causing an incredibly light image. if I manually move the black point back to were it was then the image is fine again.

Is this normal when dithering or a side effect of the ignored 1s exposures for some reason I have inserted a screen shot of what I am seeing this is after about the second or third dither. Sorry it's a bit fuzzy I was using two monitors so the screen grab was a bit small. In the image I have moved the black point back to were it would normally be in the usual histogram at this point it would normally run down to the left after the black point and be fairly level like the second half of the left side last night is the first time I have seen the two other peaks develop.

Not sure if this is a bug or has some other root cause.
Histogram.jpg
Histogram.jpg (12.1 KiB) Viewed 399 times
Cheers
Nick

BlackWikkett
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Re: Weird Histogram with Dither

Post by BlackWikkett » Sat Sep 28, 2019 2:10 pm

This is normal due to stacking and dithering increases the peaks making it more evident. This is all the overlap on the edges of the image that would be cropped out.

nexusjeep
Posts: 204
Joined: Thu Oct 18, 2018 3:01 pm
Location: Gloucestershire

Re: Weird Histogram with Dither

Post by nexusjeep » Sat Sep 28, 2019 3:01 pm

Cheers Wikkett,
At least I know it's not me or the actual software now.

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admin
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Re: Weird Histogram with Dither

Post by admin » Sun Sep 29, 2019 1:34 pm

Hi,

What is happening here is that after the dither operation, some areas near the edge of the image are no longer receiving additional data from the stacking because the stack no longer aligns with the incoming frames. When you have the default stacking algorithm selected, these areas will slowly darken relative to the rest of the frame which is receiving new data. In fact these will become the darkest areas of the frame after a wild and lead to the extra peaks you are seeing in the histogram below the normal black level. This can also happen without dithering if you are imaging unguided and the images gradually drifting.

Hope that makes sense, Robin

nexusjeep
Posts: 204
Joined: Thu Oct 18, 2018 3:01 pm
Location: Gloucestershire

Re: Weird Histogram with Dither

Post by nexusjeep » Sun Sep 29, 2019 2:36 pm

Cheers Robin at least I know it is normal now.

Thanks
Nick

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