Add option to use Sigma clipping when capturing Flat

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saguaro
Posts: 68
Joined: Tue Feb 14, 2017 9:04 pm

Add option to use Sigma clipping when capturing Flat

#1

Post by saguaro »

I typically take twilight (sky) flats, since they are convenient for me. I first turn off tracking for my mount, so that any stars visible in the frame will drift from one frame to the next, so they can be calibrated out. Currently if I use the "Capture Flat" feature in SharpCap, I believe it uses the "default" stacking method which does not allow the stars to calibrate out.

It would be great if the "Capture Flat" tool had the option to choose the Sigma clipping stacking method (similar to what the Live Stacking feature offers now on the "Stacking" tab), to allow the stars to be calibrated out of the master flat. Thanks.

-Robin
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admin
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Re: Add option to use Sigma clipping when capturing Flat

#2

Post by admin »

Hi Robin,

sadly this wouldn't work - if you use sigma clipping in a post processing tool like DSS it can look at all the frames to be stacked together, work out the mean/sigma for each pixel and then throw out the ones that are out of a certain sigma range.

SharpCap can't work like that as it would need to store every single frame so far in a live stack in memory and re-process all of them at each new frame to re-calculate means and sigmas. Instead, SharpCap uses an approximation where the first few frames (about 10) are stacked without any clipping to get a baseline estimate of mean and sigma values. Frames after this initial set are filtered and the mean/sigma values are updates on an ongoing basis. This approach means that SharpCap can run a very good approximation of a true sigma clipped stack without needing massive amounts of memory and without each frame taking longer to stack than the last. Sadly it means that your use case can't work as the stars will be in the stack through those initial batch of frames, sorry.

cheers,

Robin
saguaro
Posts: 68
Joined: Tue Feb 14, 2017 9:04 pm

Re: Add option to use Sigma clipping when capturing Flat

#3

Post by saguaro »

OK, thanks for explaining how it works. My workaround is to create the master flat in DSS (using sigma clipping) and then browse to it in SharpCap. As long as the master flat is in FITS format, this should work, correct?

-Robin
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Re: Add option to use Sigma clipping when capturing Flat

#4

Post by admin »

Hi,

yes, I think this should work as it will throw away the star in each frame due to the sigma limit.

cheers,
Robin
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