livestacking histogram jumps around more than before?

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dmccallie
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livestacking histogram jumps around more than before?

#1

Post by dmccallie »

I've noticed that recently (?) that the livestacking histogram jumps around quite a bit during the first few minutes of stacking. We show images to the public at our club's observatory, and the rapidly changing histogram can make it hard to keep a good image on the screen for the audience.

I don't recall having as much trouble getting a good image out of the histogram in the past, though perhaps my recall is bad.

We have a large aperture (30 inch, F5) Newtonian, with a ZWO 071 camera, so we can get good images on the screen within a few seconds. Our exposures are often rather short (10 seconds or so) since the tracking of our homemade mount is not perfect.

The frames are all aligning properly. I've tried both integration methods, and the result is the same - the histogram moves around so much between shots that it's hard to keep a good stretch projected to the audience. Over the course of a few minutes, the histogram stabilizes, and we are fine, but I don't recall this much early movement in the histogram in the past. I've tried it with and without background removal - that makes no difference. I am usually in logarithmic view, and usually not in zoomed view. Darks/flats don't seem to matter either.

(BTW, we love what SharpCap does for us, so this is a "good problem to have".)

Did something change in the way the histogram is calculated / displayed, or is it just my poor memory that it used to be more stable?
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Re: livestacking histogram jumps around more than before?

#2

Post by admin »

Hi,

I don't recall making any significant changes around the histogram or the way live stacking works for quite a while. Does it still happen if you take the stacking down to the simplest setup (no darks/flats, no enhancements like colour adjust, sharpening, etc). Do you have any idea of how old the version of SharpCap was that you remember not being affected by this? I would be happy to investigate, just trying to collect as much info as possible to narrow down the possibilities.

cheers,

Robin

PS. If you could do a screen recording/video of it happening and share that, it would be helpful.
dmccallie
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Re: livestacking histogram jumps around more than before?

#3

Post by dmccallie »

Hi Robin,

I've tried shutting everything else off - down to basics with no darks, flats, gradient removal, sharpening, etc. That doesn't seem to make a difference. I don't recall when the "change" seemed to occur, but several of the people in our club that I have trained have made note of it to me, so I don't think it's my imagination? We used version 3 last year, so maybe it was the transition from v3 to v4??

My naive assumption is that the mean of the stack changes a bit as each new subframe is averaged in. With the histogram in log view mode, those small changes cause the histogram peak to jump enough to the right that any stretch applied by setting the black point and mid-point lines tight against the histogram peak becomes suddenly wrong, causing the displayed image to look bad. We then either re-set the black and mid points, or click the "automatic" arrow. This makes for a jarring experience to the audience watching the livestack emerge. The image looks good, then bad, then we adjust and it's good, then bad, etc.

It just feels like this didn't happen before. We've always had to tweak the black/mid points during stacking, but I don't recall these larger jumps that make the image unstable to watch during the first few dozen subframes. Maybe in the past you were automatically readjusting the black and mid points to occupy the same relative position to the histogram peak as it moves? Just a wild guess.

I think I noted that we are using short exposures (8-20 seconds) at gain ~200 on an 071 camera. But that hasn't changed in a long time.

Next time I have it running, I will try to get a screen capture or movie. Thanks!
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Re: livestacking histogram jumps around more than before?

#4

Post by admin »

Hi,

thanks for the further info - the one thing I can think of is that if you have any image drift then you start to get areas near one or two sides of the image that stop receiving data (because they are now out of view). This can show as an additional peak near the black end of the histogram (or an extension downward of the black peak). That would have been the same in 3.2 though, I think.

cheers,

Robin
roelb
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Re: livestacking histogram jumps around more than before?

#5

Post by roelb »

I also have experienced sometimes such "weird behavior".
What I have found that this is mostly correlated to the sky seeing/transparency.
Eg. with the moon lighting up the sky it is hard to hold the histogram stacking stable.
Also when their are intermittent high veil clouds.
Just a consideration.
Roel
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Tiago Ferreira
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Re: livestacking histogram jumps around more than before?

#6

Post by Tiago Ferreira »

It happen to me exactly the same with my dobsonians when live stack only if i use any of the background subtraction or gradient removal. And after turning these features off it still happen. I solved just not using any of those 3 features at all, never. I had to reset everything to get rid of that issue.
I had high expectations on gradient removal and couldn't use it anymore. One single use and the hell start again. My 50 cents.
Robin, there's something with those features, I've reported it when you released them. The thing is, many people seem to have a good experience. I thought i was the only one and just gave up of them.
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dmccallie
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Re: livestacking histogram jumps around more than before?

#7

Post by dmccallie »

Thanks for the thoughtful replies:

Robin - we do have some drift, but in 8-10 seconds it is insignificant, yet the histogram starts shifting as soon as the second image comes in. So I don't think that's the cause.

Roelb - I had that thought too - perhaps it is sky noise. But I'm pretty sure that last year (when it didn't happen) there was lots of sky noise on certain nights, so I don't think that's it?

Tiago - that's very interesting! I don't think I ever saw this behavior before the background removal code was added. You said you had to "reset everything" to make the behavior go away. What did you do to "reset everything?" I'd like to try that experiment.

--david
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Tiago Ferreira
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Re: livestacking histogram jumps around more than before?

#8

Post by Tiago Ferreira »

dmccallie wrote: Mon May 23, 2022 11:00 pm Thanks for the thoughtful replies:

Robin - we do have some drift, but in 8-10 seconds it is insignificant, yet the histogram starts shifting as soon as the second image comes in. So I don't think that's the cause.

Roelb - I had that thought too - perhaps it is sky noise. But I'm pretty sure that last year (when it didn't happen) there was lots of sky noise on certain nights, so I don't think that's it?

Tiago - that's very interesting! I don't think I ever saw this behavior before the background removal code was added. You said you had to "reset everything" to make the behavior go away. What did you do to "reset everything?" I'd like to try that experiment.

--david
I unplugged everything and restarted the laptop. To only restart the camera didn't work but i think i solved it once buy restarting sharpcap and also unplugging the usb from the camera. I did loads of tests specially with the gradient which was what i had more expectations on so i can't remember more than this. As EAA is my main use of sharpcap, to have to adjust so many times the histogram while watching an object was not for me. But it was doing it's job removing gradient, but with me playing the cat and rat with it. I almost had no time to save as seen, it quickly bumped all the way.
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Re: livestacking histogram jumps around more than before?

#9

Post by admin »

Hi folks,

this is all helpful - the more info we can collect on what might or might not be causing it will help me test out the possibilities and hopefully find a solution

cheers,

Robin
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Re: livestacking histogram jumps around more than before?

#10

Post by admin »

Hi,

thinking about this a bit more, it occurred to me that you might be turning background correction/gradient removal/dark subtraction on or off, or changing settings, after a stack has started. Doing that is definitely going to cause the histogram to start to move around, as each new frame after the change will alter the ratio of frames with the original background brightness (before the change) to frames with the new brightness (captued after the change). There is no real way to fix this behaviour - the right thing to do is to restart the stack if you make changes to those controls.

cheers,

Robin
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