Live stacking question

Discussion of using SharpCap for Deep Sky Imaging
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Live stacking question

Post by mike-va » Sun Jul 08, 2018 5:05 pm

I searched for "live stack" and "live stacking" but got no results because "live stack are too common words." So I'll ask the question, even if it's been answered before.

The user manual indicates that individual frames are continuously added to the result during a live stack. Doesn't this cause the light regions eventually become too bright?

I understand why one might want to add a series of long-exposure frames of a faint DSO to make a "pretty picture" later, but for EAA, don't we want to stop before the target becomes too bright?

The MallinCam software I'm currently using with my DS16C has what is called a "rolling stack," where the last N frames are added together to produce the result. I can specify N, so typically we're adding the last five or 10 frames. This results in a nice-looking image that's always produced from the N most recent frames.

Does SharpCap have such a "rolling stack" feature? If not, what prevents a live stack from becoming too bright?

Or, more to the point, how does SC implement EAA so a good-looking, up-to-date image is always displayed?

Thanks for educating this SC newbie.

BTW, I am seriously considering replacing the DS16C with an ASI1600C Pro, and using SC instead of the MallinCam software.

--- Mike

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Re: Live stacking question

Post by admin » Mon Jul 09, 2018 6:02 am

Hi Mike,

SharpCap keeps count of the stack frame brightness using data structures that can hold a value of up to ~2 billion. Each frame has a maximum value of ~64000 (if you are using a 12/16 bit mode), so you can have about 32000 frames added before the bright parts get too bright. That's about 9 hours at 1 second exposures or an awful lot longer if you use longer exposures.

When SharpCap displays the frame it looks for the highest value in the stack first and makes that white and scales all the others relatively.

Now what this means is that the effect of live stacking in SharpCap is not really to make the image brighter (the image brightness will stay roughly the same throughout the process), it's to reveal more detail with less noise as the number of frames builds up. What you often find is that you can increase the intensity of the image level stretch as the stack progresses and the noise reduces to bring out more faint detail.

Hope this helps,


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