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Relationship between FWHM of the individual frames and FWHM of the final stack [Deep Sky] Processing techniques · Emilio Frangella · ... · 10 · 236 · 1

_The3D_ 2.86
Emilio Frangella
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I am trying to understand what's the relationship, if any, between the sharpness of the individual frames versus the sharpness of the final stacked RAW through WBPP.
My latest image https://www.astrobin.com/full/q1t9sq/0/ has an average FWHM of the individual frames, measured before stacking through subframe selector, of 1.85" (around 4px at 0.46 arcsec/pixel). After the stacking process, the resulting FWHM is around 4.6px, or 2.15 arcsec. I am trying to understand if i am losing sharpness during the stacking due to some internal process (maybe imperfect star alignment?) or this behavior is to be expected. To be noted that this discrepancy is somewhat more noticeable with my C11 (which was used to take that image) versus my RC (stacks coming from the RC data are usually much closer in FWHM to the average calculated using SFS).

Does anyone have insights on the topic?
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_The3D_ 2.86
Emilio Frangella
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One of my guesses is that compared to the individual frames, there are many more objects (like distant galaxies) of irregular shapes present in the image that might be mistaken by the algorithm for stars, skewing the result. I wonder if that could be the case.
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andreatax 9.89
andrea tasselli
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No, the difference is due to the alignment (and consequential resampling) process. Beautiful image, btw.
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Gondola 8.11
Tony Gondola
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There's a lot going on in the stacking process. I would not expect that math to work out perfectly on this.
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_The3D_ 2.86
Emilio Frangella
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thanks Andrea. Is this behavior to be somewhat expected? does it factually impact the overall sharpness of the image, and if yes, can it be reduced? i think 0.4 arcsecs of loss in resolution is quite meaningful.
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AndreVilhena 4.72
Andre Vilhena
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I believe the reason for that would be that the stacking will lower the noise floor and make the fainter sections of the stars more visible, thus enlarge then. I think the drawing below helps explaining:
image.png

In your subframe, since noise level is higher, the algorithm is seeing the star "cut" at line 1 but when you stack the subframes the noise level drops to 2, for example, and the algorithm see more of the star.  In the first case, the FWHM will be given by the distance between the two red lines and, in the second case, between the two green lines.

The drawing is not 100% accurate as in reality the "cutoff" is not so abrupt but I think you get the ideia.

In addition, if registration is not 100% accurate (and usually it isn't in the whole FOV), stars in each subframe might be spreaded a bit in the final stack. But I think generally the impact of this is not so large.

Additional note: if you take a 5 sec subframe and a 20 sec subframe for example, you'll notice the FWHM of the latter is larger than the first - that is related with the PSF drawing above.

Cheers,
André
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andreatax 9.89
andrea tasselli
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Emilio Frangella:
thanks Andrea. Is this behavior to be somewhat expected? does it factually impact the overall sharpness of the image, and if yes, can it be reduced? i think 0.4 arcsecs of loss in resolution is quite meaningful.

*Yes, it does. It always was thus, by and large at around 10% mark. You gain, however, massively on SNR which allows for deconvolution to be applied and thus resolution regained aand then some...
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_The3D_ 2.86
Emilio Frangella
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andrea tasselli:
Emilio Frangella:
thanks Andrea. Is this behavior to be somewhat expected? does it factually impact the overall sharpness of the image, and if yes, can it be reduced? i think 0.4 arcsecs of loss in resolution is quite meaningful.

*Yes, it does. It always was thus, by and large at around 10% mark. You gain, however, massively on SNR which allows for deconvolution to be applied and thus resolution regained aand then some...

got it, so to be expected somehow. So this brings up the next question, why is this gap much larger with my C11 compared to the RC. Playing the guessing game again, since SCTs optics usually suffer from "fatter" stars, sounds reasonable to me that the effect suggested by @Andre Vilhenamight be even stronger, hence the difference. Would be interesting to understand if we can maybe refine the star alignment configuration to try and minimize this delta, and see if it actually makes any noticeable difference in sharpness.
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Ricksastro 1.51
Rick Krejci
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My theory would be that the signal of the Full Width part of FHWM is increasing as you stack so many frames, but the Maximum is not as much since some of the stars are saturated.
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andreatax 9.89
andrea tasselli
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Emilio Frangella:
got it, so to be expected somehow. So this brings up the next question, why is this gap much larger with my C11 compared to the RC. Playing the guessing game again, since SCTs optics usually suffer from "fatter" stars, sounds reasonable to me that the effect suggested by @Andre Vilhenamight be even stronger, hence the difference. Would be interesting to understand if we can maybe refine the star alignment configuration to try and minimize this delta, and see if it actually makes any noticeable difference in sharpness.


SCTs suffer from a minor level of spherochromatism (as most cats do) which isn't there for purely reflective optics (end flatteners/reducers do not add meaningfully to that), so your scattering of FWHM is larger because, while the central wavelength is well within boundaries (i.e., diffraction limits) this may not be equally applicable to the shorter wavelength where seeing is most effective in broadening the wings of the FWHM profile. I'm talking here about L values rather than specific color filters but it would be instructive to look at how they distribute between R, G and B.

I don't think that the reduction of the noise floor makes any difference to that, btw.
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Gondola 8.11
Tony Gondola
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…..and don't forget that the stars in each subframe are not perfect. This is averaged out in the stacking process but adds to the degradation.
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