Color balance and histogram anomaly

Somewhere to ask questions about the best way to use SharpCap
Post Reply
descott12
Posts: 32
Joined: Mon Dec 17, 2018 2:32 am
Location: Charlotte, NC USA

Color balance and histogram anomaly

Post by descott12 » Mon Jan 07, 2019 4:54 pm

Hello,
A few issues arose in my last session a few nights ago. I posted these questions in another thread but I think they got lost in the clutter...

1. In the M42 screenshot, the histogram looked good until about 16 minutes then a hump developed at the left side that I kept having to clip off my moving the black level to the right. Note also the bluish background tinge. My focus appears a bit off at that point but I think clouds had rolled in. What is the cause of the hump and is that related to the background bluish coloration? I seem to have that issue in most of my stacks. The background is not black but usually has blue tinge.

2. Running man - The color balance was way off and I noticed that the RGB peaks were not overlaying as I think they should. Should they pretty much overlay? and if yes, what causes them to diverge? I tried using the auto-color correction buttons but they often made it worse or caused the problem to begin with. Adjusting the vertical RGB sliders really didn't have the effect I wanted and a very small decrease in blue, for example caused the image to turn alien green.

3. There are three histograms and they seem to have subtly different effects/uses?
> The one used during live stacking - can we switch to logarithmic view? I don't see a way to do that
> The one in the lower right corner - how is this different than the live stack view?
> The one that you get to from the tool bar that has the Brain - I understand the Brain function a little and this one does not seem to be available during live stacking. Maybe used just for a few initial shots to setup the exposure?

Thanks in advance
Attachments
Very blue.png
Very blue.png (1021.87 KiB) Viewed 459 times

User avatar
admin
Site Admin
Posts: 2959
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Color balance and histogram anomaly

Post by admin » Mon Jan 07, 2019 8:50 pm

Hi,

so it could be possible that something like a little high cloud arriving could have brightened up the background during your imaging session causing the histogram shape to change as the new frames being added had a slightly brighter background.

On the histograms, the one in the live stack view shows the state of the stack with no colour or stretch corrections applied – i.e. the stacked data with only dark and flat corrections if those are in effect. The histogram on the right-hand side shows the state of the image as it is being displayed i.e. it includes the effects of any colour or stretch adjustments being applied in the live stacking histogram panel. So, the simple way to colour balance your image here would be to adjust the colour sliders in the live stacking panel in till the 3 peaks in the right-hand panel histogram coincide.

Hope this helps, Robin

descott12
Posts: 32
Joined: Mon Dec 17, 2018 2:32 am
Location: Charlotte, NC USA

Re: Color balance and histogram anomaly

Post by descott12 » Mon Jan 07, 2019 9:46 pm

Perfect. Thanks Robin.

descott12
Posts: 32
Joined: Mon Dec 17, 2018 2:32 am
Location: Charlotte, NC USA

Re: Color balance and histogram anomaly

Post by descott12 » Mon Jan 07, 2019 9:53 pm

Robin,
One other question. For some reason my M42 image isn't posted but the problem is visible in the live stack histogram for Running Man.
Initially the histogram dropped off the zero near the left side and the peak was the correct shape. But as the stacking proceeded, the baseline of my histogram elevated as can be seen to the left of the black line. What does that mean and I how would I address that? I know it should not look like that.
As this started to appear, I noticed that the background kept getting "less dark"
Thanks

User avatar
admin
Site Admin
Posts: 2959
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Color balance and histogram anomaly

Post by admin » Wed Jan 09, 2019 5:01 pm

Hi,

it took me awhile to remember that I've seen this growing histogram in the black area before and worked out what the cause was the last time round.

What is happening is that as a misalignment or rotation builds up between the initial frames in the stack and the current frames stack, some of the pixels near the edge of the stack are no longer having data added to them. This is because when SharpCap rotates or offsets the image to correct for any misalignment or rotation, the areas that are pulled into the frame by the rotation or offset are black. In general this is not anything that you need to worry about – you can actually see it happen in a without using a telescope if you use the deep sky test camera in SharpCap, turn on the random rotation option and enable live stacking. You will see that dark areas build up around the edge of the stack due to the continuous rotation used in this testing mode.

I'm not sure however sure how this could lead to the background of the stacked image starting to get less dark – that would sound to me more like a symptom of my initial suggestion i.e. perhaps some thin cloud had arrived it was leading to a slightly brighter sky.

Maybe if you try again to share your image showing the problem there may be more to learn. I believe you may have bumped into a bug in the forum software which can make the 2nd and subsequent image attachments vanish you use the preview option before posting.

Hope this helps, Robin

User avatar
turfpit
Posts: 813
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Color balance and histogram anomaly

Post by turfpit » Wed Jan 09, 2019 6:00 pm

a bug in the forum software which can make the 2nd and subsequent image attachments vanish you use the preview option before posting
I can confirm this. When adding more that 1 image to a post, do not do a Preview before a Submit. Doing Preview will cause images to disappear.

Dave

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest