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Old 27-09-2021, 11:32 AM
Emuhead (Andrew)
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Choosing optimal reference frame for maximum overlap of all subs

Hi guys,

1) So have shot over 4 nights, same target, there is some rotation in the subs so im looking for a way to simply stack all the subs, then determine which single sub aligns 'the most' with all of the other subs and then use that as a reference image for registration & local normalisation. Is there a clever way?

2) Multi night imaging - avoiding rotation when you have to remove your imaging train from your scope... whats the best way to align this up before you start the imaging run so you can avoid the above?
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Old 27-09-2021, 11:34 AM
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multiweb (Marc)
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Generally you would select the sub with the tighter stars to align to. Offset is good and will work to your advantage during data rejection. Field rotation not so well if it's visible within one sub. Otherwise it's good too.
When I go back to a target days later or years later I load the last sub, pick 3 bright stars in nebulosity then overlay over the markers while doing a focusing run. That gets you close enough.
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Old 27-09-2021, 11:45 AM
Emuhead (Andrew)
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Thanks Marc and I do tend to follow that method normally and its been great, but for this data Im looking for a way to determine which sub aligns the most often (rotation wise) with the highest number of other frames to get the best result from the local normalisation process (so it only fills in black sections/missing data on the least amount of subs).
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Old 27-09-2021, 11:49 AM
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multiweb (Marc)
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Quote:
Originally Posted by Emuhead View Post
Thanks Marc and I do tend to follow that method normally and its been great, but for this data Im looking for a way to determine which sub aligns the most often (rotation wise) with the highest number of other frames to get the best result from the local normalisation process (so it only fills in black sections/missing data on the least amount of subs).
Then I'd load the aligned stack in a program that can blink through the stack and look for the smallest common overlap and select the best sub this way. It's a manual process though. When you normalise it's important to pick carefully an area that always has data or you'll get false readings. Usually an area closer to the center of the field.
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Old 27-09-2021, 12:00 PM
Emuhead (Andrew)
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Not knowing a program that would do that tbh so for now ill just manually use Blink and try to 'best guess' it.

Is there a best practice for easily working with rotation to be consistent with previous nights rotations? At the moment im just eyeballing it from new previews vs previous nights subs, and whilst that gets me 90% of the way there, would be nice to know its centered & rotated as close to perfect as possible with the least amount of time spent. Obviously trying to avoid an actual $1000 rotator.
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Old 27-09-2021, 12:01 PM
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DiscoDuck (Paul)
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In PixInsight, you can register them to the union of all the images I believe with Register Union Separate as the working mode in the StarAlignment process. You'd get each image then with a black border. Then you could stack these and crop the result as you desire.
(The pixel rejection algorithms may freak out at multiple black images at some pixels and reject the true signal, but that's ok, as you're only interested in the intersection of your images).

Edit: Just had a look and it may be messy to do it like this ... at least more than my first guess .... as I don't think you can do all images at once but have to kind of add them one by one and keep updating the reference image to the new one until all accumulated?? Not sure. But sort of create a mosaic from them all, then register the individual images against that mosaic and then stack them. Hmm. Not easy. Perhaps a PI wizard has a slick way to do it.

Or alternatively, run StarAlignment in the normal mode with a randomly chosen reference image, take the average of the computed offsets for each image to get the center of mass, and pick as a reference image the frame closest to that average offset. Then run again with that as the reference.

Last edited by DiscoDuck; 27-09-2021 at 06:25 PM.
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Old 27-09-2021, 12:11 PM
Emuhead (Andrew)
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That sounds promising, trying these now.

Thanks
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Old 27-09-2021, 12:58 PM
Startrek (Martin)
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Andrew
Give Deep Sky Stacker a try ( it’s free, very intuitive and IMO one of the best stacking software around )
I’ve stacked frames from various nights that are way out of alignment and DSS stacked fine . I just cropped the perimeter afterwards in my processing software Startools
Cheers
Martin
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Old 27-09-2021, 02:08 PM
Emuhead (Andrew)
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Thanks Martin, I'll try that also.

Perhaps I'm overthinking it, just pick a sub that has the framing I'm looking for, and let stacking do its best trying to provide an even looking final image, without a noticeable degradation or darkening of the corners (that were rotated and therefore have less data to work with).

I'm trying not to crop actually, as i have the framing im after, im just looking for the best way to fully keep that whilst also making use of some rotated frames in the stack.
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