Exploring the limits of DSO sharpness

[timh]

The galaxy images that I posted recently were all derived from captures that suffered from rather poor star shape with average Eccentricity values ranging from about 0.5 up to 0.7. Neverrtheless, RC BlurExterminator appeared able to correct the problem very effectively.

Sometimes though you don't realize quite how poor the images that you have been working with were until some much better ones come along. Finally, with better balancing in 4D of my bulky VX12 Newtonian -- with various counterweights attached (thanks for the advice from Astroshed's youtube channel) -- on the CEM70 mount and a night of relatively good seeing I have finally been lucky enough to capture some good images of M51 at low FWHM and with round stars at a relatively high level of brightness and SNR.

Again the set up was a VX12 12 inch telescope F4 on a CEM70 mount with PDS guiding via a top mounted 80 mm refractor. The camera was an ASI294 MM at 46Mb BIN1 with 2.315 u pixels giving an image scale of 0.406 arcsec/ pixel. Subs were 10s at gain 280 using a ZWO UV/IR filter only - calculated to contribute < 5% read noise at the average measured sky brightness (~4 electron / s / pixel).

The integrated image obtained was derived from 601 x 10s subframes - the FWHM was estimated as 1.60 and the Eccentricity as 0.395. This was processed with RC Blur Exterminator in PixInsight - firstly via correction only - and then sharpening - non stellar by 0.6X and stellar by 0.33X. (not at all sure how BXT works but from observing what happens I suspect that the first correction step is a clever implementation of a localised form of deconvolution which also retores star symmetry and that it is the following non-stellar sharpening step that is AI driven and that makes use of a neural net of concensus DSO subfragment shapes?)

Anyway - finally having this relatively good image of M51 as a starting point I was able to a) see just how far BXT can take DSO resolution in comparison with the NASA/ ESA HST image of 2005 and b) use convolution to blur and down sample the original image in order to better define which parameters of input images are most critical to getting a very high quality image out at the end following BXT processing.

The broad conclusions that I drew from the data (otheres may disagree) were --

1) The BXT-improved image of the already quite sharp 601 x10s input image (FWHM 1.6, ECCEN 0.395) was remarkably sharp and looks to have delivered a lot of the detail of the HST image (the HST image was obviously far better -- but it did slightly exceed my budget at $16B thus far!). From pixel peeping I would estimate the resolution at less than an arcsec (i.e. starting to approach the 2X Nyquist sampling imposed limit for 0.406 arc sec / pixel) .

2) As would be expected -- BXT delivered great improvement to the image sampled at 0.406 arcsec/ pixel, a good deal less improvement to the image after it was first downsampled to 0.81 arcsec/ pixel and (not shown) no improvement at all after further downsampling to 1.6 arcsec/ pixel (the starting image already exhibited an average FWHM value of 1.6 so no improvement was possible).

3) Again as expected the sharpness of the starting image is a critical determinant of what Blur Exterminator can deliver. Symmetrical gaussian blurring of the starting image from an FWHM of 1.6 up to 1.92 (with an associated slight improvement in Eccentricty) led to quite a dramatic decrease in the level of detail of the final image delivered by BXT. The image actually looked smoother with better SNR - but the finer detail had gone.

4) Finally, assymetrical Blurring leading to only a slight increase in FWHM and a relatively large increase in Eccentricity from 0.39 to 0.52 was a bit more surprising. This distortion did lead to a loss of detail in the final image delivered by BXT but it was actually quite slight and subtle.

So the BXT correction function does seem pretty effective at compensating for at least some forms of image distortion - at least up to an Eccentricity of ~0.52 -without too much detriment to the quality of the image that it finally delivers.

Tim

[turfpit]

Tim

Interesting write-up. The HST image shows how much we have to battle with the atmosphere.

Dave

[Menno555]

Again a great write up Tim!
And it's also so cool to see and read how we all approach our hobby in a different way.

Because I do kind of the opposite of you : both with capturing and processing, I just "fiddle around" with things until I am satisfied. Following my gut feeling so to speak, using what I consider logical. No idea or calculations whatsoever about FWHM, sky quality, eccentricity, and so on.
I had online discussions with people who who "blamed" me of not being scientific, not doing real astronomy because I don't use/understand certain things. Or just plain didn't believe me cause, and I quote: " .. your captures only can be done with everything perfectly calculated!".
Guess my logical gut feeling does all that

So for me your write-up is very interesting and educational. And also cool to compare (but not for shapness ): I once did a blow up of the M51 core. Did a 3 x drizzle stack in DSS and used SiriL and Photoshop. See the result below. This with an 8" Meade LX200 and in Bortle 7 skies.

Menno

kern_m51.jpg (487.49 KiB) Viewed 5175 times

[timh]

Thanks and yes I quite agree Menno. A whole lot of quite different interests reflected on this site --- SharpCap has become like a Swiss army knife that now caters very well for the lot --- Lunar, solar, planetary, EEA -- following variable stars. You - I would say - tend to find stuff that is inherently interesting or that can be differently presented in some way - e.g. looking at the expansion of M1 over 50 years , experimenting with IR imaging , finding HH objects or a neglected galaxy etc. It's a good and fun approach. I probably have the opposite tendency to try and start from theory first -- because I just rather like the challenge of grappling with the Physics.... with the sometimes dull result probably that I can sometimes image the same thing for 6 months just trying to improve some aspect of the set up or processing in some way. But it is remarkable how quickly not only one's own understanding but also the technology itself moves on..

I remember that image that you posted of the M51 core and thinking how much impressive detail it showed at the time - pretty much state of the art from B7 probably. Short frame 'semi' lucky imaging and just ordinary deconvolution could probably take you a bit further -- but really I do think that RC Blur Exterminator now represents a step change in the clarity that amateurs can bring to DSO imaging. Also a change that means --if one is to use the software routinely - has consequences for the front end and image capture. For example it means that 'oversampling' is not really oversampling anymore and is actually necessary to get the best out of the tool -- etc.

I extracted the luminance from your image and compared it with my image both before and after BXT processing -- mine (unprocessed at lower contrast) starts off comparable -- but then what a difference BXT makes.

But Dave is quite right -- what a gap there is also between even the BXT improved image and the HST image. I have a feeling that this is far as the sharpness improvement for DSOs can go for now --until of course amateurs can control remote telescopes in space or on the moon rather than Namibia or wherever -- probably will happen one day? !

Tim

[turfpit]

I have a feeling that this is far as the sharpness improvement for DSOs can go for now --until of course amateurs can control remote telescopes in space

Tim

I don't fancy the hourly rate for a telescope in space ....

Dave

[turfpit]

In the July 2024 issue of Astronomy Now magazine, there is a 4 page review of Blur Xterminator v2.0 in the Imaging Masterclass series.

Dave

[timh]

Thanks Dave, will have a look at that!