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tech / sci.astro.amateur / Re: Have to wonder if computer-derived images are real

SubjectAuthor
* Re: Have to wonder if computer-derived images are realMartin Brown
+- Re: Have to wonder if computer-derived images are realChris L Peterson
`- Re: Have to wonder if computer-derived images are realMartin Brown

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Re: Have to wonder if computer-derived images are real

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From: '''newsp...@nonad.co.uk (Martin Brown)
Newsgroups: sci.astro.amateur
Subject: Re: Have to wonder if computer-derived images are real
Date: Wed, 2 Feb 2022 16:48:18 +0000
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 by: Martin Brown - Wed, 2 Feb 2022 16:48 UTC

On 01/02/2022 01:51, RichA wrote:

> https://phys.org/news/2022-01-universe-sharper-focus-algorithms-supercomputers.html

Depends what you mean by real. The trade off is between resolution and
signal to noise. Those raw images all look to have plenty of signal to
noise and so are amenable to being processed by deconvolution.

Effectively the software asks the question "what is the most uniform
positive image that is consistent with the observational data?".

It knows how the image is affected by solar wind and sparse
interferometer baselines so it can compute forward from various trial
images and obtain a self consistent solution.

In the cases where a comparison made using a later bigger telescope has
been possible the agreement was remarkably good. Cass A 5GHz from the
Cambridge 5km telescope with Maximum Entropy decovolution vs VLA in full
multiple A,B,C & D raw image being one such example. The latter was
latter processed with the same algorithms to a higher resolution:

https://www.nezumidemon.co.uk/astro/radio/cassa.htm

I think the final definitive version that made it onto SPL is too soot
and whitewash for my taste but then I may be biassed.

https://www.sciencephoto.com/media/332250/view/radio-photo-of-supernova-remnant-cassiopeia-a

I would be wary of anyone claiming to push super resolution much beyond
3 or 4 times in linear dimensions unless their data was exceptionally
low noise. The bright unresolved point sources in some of those radio
galaxies make them very amenable to some of the heuristics and tricks.

They will also have images made at other wavelengths of the same
objects. It is standard to compare one with the other. You can often
extrapolate what something might look like between frequencies.

--
Regards,
Martin Brown

Re: Have to wonder if computer-derived images are real

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From: clp...@alumni.caltech.edu (Chris L Peterson)
Newsgroups: sci.astro.amateur
Subject: Re: Have to wonder if computer-derived images are real
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 by: Chris L Peterson - Thu, 3 Feb 2022 14:59 UTC

On Wed, 2 Feb 2022 20:17:46 -0800 (PST), RichA <rander3128@gmail.com>
wrote:

>On Wednesday, 2 February 2022 at 11:48:24 UTC-5, Martin Brown wrote:
>> On 01/02/2022 01:51, RichA wrote:
>>
>> > https://phys.org/news/2022-01-universe-sharper-focus-algorithms-supercomputers.html
>>
>> Depends what you mean by real. The trade off is between resolution and
>> signal to noise. Those raw images all look to have plenty of signal to
>> noise and so are amenable to being processed by deconvolution.
>>
>
>I remember how God-awfully bad the pre-fix "deconvoluted" Hubble images were.
>Of course, computers weren't what they are now.

They may have been ugly, but they were not inaccurate. That is often
the case for deconvolved images.

Re: Have to wonder if computer-derived images are real

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From: '''newsp...@nonad.co.uk (Martin Brown)
Newsgroups: sci.astro.amateur
Subject: Re: Have to wonder if computer-derived images are real
Date: Fri, 4 Feb 2022 09:32:10 +0000
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 by: Martin Brown - Fri, 4 Feb 2022 09:32 UTC

On 03/02/2022 04:17, RichA wrote:
> On Wednesday, 2 February 2022 at 11:48:24 UTC-5, Martin Brown wrote:
>> On 01/02/2022 01:51, RichA wrote:
>>
>>> https://phys.org/news/2022-01-universe-sharper-focus-algorithms-supercomputers.html
>>
>> Depends what you mean by real. The trade off is between resolution and
>> signal to noise. Those raw images all look to have plenty of signal to
>> noise and so are amenable to being processed by deconvolution.
>>
>
> I remember how God-awfully bad the pre-fix "deconvoluted" Hubble images were.
> Of course, computers weren't what they are now.

They were not pretty in the classical sense but they were accurate for
scientific purposes. The main problem was that the algorithms back then
had no way to suppress ringing from stars embedded in nebulosity.

The positivity constraint doesn't work on a raised baseline.

They worked incredibly well for star fields on dark sky but not for more
complex objects. Remember that the codes used and also to diagnose the
faults in the myopic HST came from radio astronomy and were originally
done to figure the Jodrell Bank dish during its refurbishment.

It is that very same procedure that they are doing now to commission
Webb - aligning the panels so that they are all accurately in phase.

--
Regards,
Martin Brown

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