Using darks strange histogram

Discussion of using SharpCap for Deep Sky Imaging
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Gamewarden
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Joined: Wed Mar 25, 2020 6:53 pm

Using darks strange histogram

#1

Post by Gamewarden »

Folks

Using a Celestron 6 SE and ZWO 294 camera with Sharpcap. Whenever I use darks with Sharpcap and my colour 294 camera 4 colour lines always start off to the right handside of the histogram and go diagonally to the black side of the histogram about halfway up, no distinct hump so it makes it difficult to adjust the black slider to the left of the hump. Sorry about the technical lanquage.

Am I clipping data at the black level? The histogram is normal if no dark is applied,

Thanks Mark
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admin
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Re: Using darks strange histogram

#2

Post by admin »

Hi Mark,

if you have no significant sky brightness then you can end up clipping the black level when using darks (or if the darks don't quite match somehow - different brightness setting maybe?). If it isn't that then it would be helpful if you could post a screenshot :)

cheers,

Robin
Gamewarden
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Re: Using darks strange histogram

#3

Post by Gamewarden »

Thanks Robin Next time out I will post screenshot

Thanks again Mark
Gamewarden
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Re: Using darks strange histogram

#4

Post by Gamewarden »

Live stack.jpg
Live stack.jpg (598.41 KiB) Viewed 188 times

Robin

Here is a screenshot of my darks
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Re: Using darks strange histogram

#5

Post by admin »

Hi,

OK, there are a few things you can do to try to improve this

1) Update to the very latest SharpCap version from the downloads page - I recently added code to include more detail in the graph down at the black end, which will make the graph display somewhat smoother.

2) Check that the histogram is well separated from the left hand side when capturing dark frames - move the peak to the right by increasing brightness value if necessary so that the left hand side of the peak has clear space between it and the vertical axis of the histogram. Especially important if using hight gain.

2) Check that the settings used for taking dark frames are exactly the same as for the light frames (gain, exposure, temperature, colour balance and brightness value at the very least). If these do not match then the darks may over-subtract and give you this sort of issue

4) If all else fails, turn up the brightness a bit when capturing light frames (compared to the dark frames). This will increment the brightness of the light frame a bit (same constant amount at every pixel), moving the histogram peak away from the black point.

Hope this helps,

Robin
Gamewarden
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Re: Using darks strange histogram

#6

Post by Gamewarden »

Thanks Robin appreciate all the time you take to answer questions. Mark
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