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Stacking multiple nights from different light pollution levels (DSS)

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#1 bigopapa



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Posted 05 July 2022 - 05:26 AM



So I got around 40 minutes of data on NGC7000 at a Bortle 4 location. Now, I took another 3 hours from my Bortle 9 balcony, with very different settings (except for focal length, which I kept as close as I could).


I tried stacking both of the nights together, but I'm not sure what to put at the star detection threshold. The exposures from Bortle 4 have much more stars than the exposures from Bortle 9, so even if DSS is detecting 1000 stars on the exposures from Bortle 4 it barely gets 30 on the exposures from Bortle 9.


This is what I got after stacking:

Two night stack
When stretching the part with the nebula, I get less detail and more noise than just the exposures from Bortle 4 (40 minutes compared to 3 hours). I'm not sure what caused this separation of the two sets inside the stack, but I'm thinking that maybe I recalled the focal length incorrectly and shot more zoomed in in the Bortle 9 exposures?


Is there a way to set different detection thresholds for different groups? Should I stack them separately and stack both stacks together (if that's even possible)?



#2 Tapio


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Posted 05 July 2022 - 06:40 AM

For stacking different night imaging I suggest you use file groups.

#3 james7ca


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Posted 05 July 2022 - 06:53 AM

I don't know about DSS, but your situation suggests that it might be a good time to use PixInsight's free trial period to handle such issues. PixInsight has even introduced a new weighting method that determines each sub's quality by using a large number of point spread function measurements of the stars. So, it can automatically weight and optionally reject subs based upon those measurements which will take into account such factors as focus, seeing, and sky background levels.


That said, PixInsight can be rather fiddly when it comes to finding a sufficient number of stars to do its registration. But, there are adjustments that can be made individually for different groups of subs which can then be stacked together in a single, final integration step (i.e. if you need to you can register subs separately and then combine all of those into one integration). However, PixInsight's quality rating is done during either the calibration (mono) or debayering (color) phase of the processing so all of your subs can calibrated and weighted prior to registration.


By the way, when I click on your image preview I get a message that says I don't have permissions to view the image.

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