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Old 23-08-2021, 07:16 PM
DIYman (Doug)
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Compromising when selecting frames to stackages

Here is a question which is causing some debate among my astro friends.



Is it better to include in stack images those frames that do not meet a FWHM standard just for the sake of increasing the number of stackable frames? The rationale here is that the end result will be better than just using a limited number of a really good frames?



I have not been in astro-imaging long enough to form a view.
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Old 23-08-2021, 10:25 PM
glend (Glen)
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Run the image files through Sequence Generator Pro's, Image Grader, that makes a fair assessment of the quality of the batch. Image Grader can scan whole files of subs, and compare by filter. The subs will be given a score, relative rank, variance from the standard deviation, etc. You can always view a few to determine where you want to cut off the inclusions.
Usually you want files with good signal strength. Inclusion of lower quality subs should be avoided in my opinion.
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Old 24-08-2021, 06:08 AM
RyanJones
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Hi Doug,

Although it’s hard to bring yourself to do having spent hours collecting subs, including sub par subs has no benifit. The universe isnt going anywhere any time soon so if you find you have to cut down your subs because they aren’t up to standard so be it. If that leaves you with not enough subs to give you a good SNR in the final image then you just have to spend another session on the target. Even if you end up throwing out an entire session, you won’t be the first or last to do it. As Steven Covey says “ begin with the end in mind “. Note what caused you to have to cut subs such as bad seeing, rushing your PA, not refocusing when there has been a significant change in temperature throughout the session etc. If you learn from it, it’s not a wasted session.

Cheers

Ryan
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Old 24-08-2021, 06:44 AM
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multiweb (Marc)
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Depending on how much data you've collected it's always a compromise between noise and sharpness. Not an issue if you have a mountain of data you can be picky. Otherwise season to taste. If I have a limited amount of subs I tend to be greedy and only reject the horrible ones. I'd rather have something a little blurrier but smooth rather than noisy.
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Old 24-08-2021, 07:57 AM
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gregbradley
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Subs can be substandard for various reasons. FWHM is only one which would indicate the focus went off, the guiding got worse or was wind affected, clouds came over, dew formed, you bumped the mount in the middle of an image being taken etc etc.

Subs affected by cloud will generally degrade the stack noticeably and give odd background colours that are hard to remove.
Slightly worsening focus if bad can damage a stack, If slight it will a bit. You can always compensate with some deconvolution. Out of focus in one colour can cause coloured rings around stars in that colour.

But for best possible resolution yeah be somewhat strict on the standard.

It comes down to how hard you want to work to get the best possible image under the circumstances of clear skies or clouds, available time, position of the object etc.

Best subs will generally be when the scope is near the zenith.

Greg.
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Old 24-08-2021, 08:59 AM
DIYman (Doug)
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Thanks Ryan for the good advice.
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Old 24-08-2021, 09:00 AM
DIYman (Doug)
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Thanks to everyone who responded to this post thereby helping to resolve our issue.
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Old 25-08-2021, 07:39 PM
Startrek (Martin)
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I manually review ( visual observation) my captured FTS subs using ASTAP prior to stacking in DSS
I open the first frame and zoom in to around 150 to 200% and adjust histogram to achieve good resolution , brightness and contrast then initially check star size and shape for bloating , irregularity or tails etc.. on 2 or 3 larger stars in the field of view as I joggle through each sub
I pick up subs affected by high cloud, guiding issues etc.. then record those to be discarded
Then I zoom out to normal frame view then joggle through each sub to check for satellite trails, passing Aircraft and other gremlins.All affected subs are then discarded.
I generally do my own grading of “borderline” subs based on experience
Most of my imaging sessions ( 2 hours up to 8 hours ) I’m discarding only 5 to 10% of the captured data.I try to avoid imaging under patchy clouded skies etc... too frustrating
This manual reviewing system seems to be time consuming ,laborious and deficient but I can review say 60 subs in a few minutes once data is loaded with a high level of confidence in the quality of the data ready for stacking. DSS then will register and stack the reviewed data based on the best 90 or 95%.
This system has worked fine for a long time , might be old school but it works fine

Cheers
PS I used a similar system years ago when I started out in AP using a DSLR ( so reviewing CR raw files ( subs) instead of FTS files now )
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Old 28-08-2021, 12:11 PM
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The_bluester (Paul)
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I have found that if you have a decent number of subs, the harder an eye you look at them with when rejecting subs, the better the end results. I don't stress too much about sat trails as outlier rejection generally gets rid of them nicely, but out of round stars from wind gusts or other tracking issue or poor contrast or bigger HFR from light clouds, in the bin!
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Old 28-08-2021, 04:52 PM
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Bassnut (Fred)
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If you use CCD stack and have many subs, the least tedious way is to wait till the data reject step and just delete the ones with the most red (red indicates deviation from the average of all the other subs). you'd be surprised at how many subs look great visually but are actually crap. generally, though, it's best to leave marginal subs in, the reject algorithm will only add good data.

Last edited by Bassnut; 28-08-2021 at 05:24 PM.
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