The six-pointed stars we see in all JWST images are rather
disturbing, yet, it should be easy enough to filter them out.
Why don't they?

Jeroen Belleman

On Mon, 16 Oct 2023 23:49:17 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

>The six-pointed stars we see in all JWST images are rather
>disturbing, yet, it should be easy enough to filter them out.
>Why don't they?
>
>Jeroen Belleman

Why disturbing?

On 2023-10-16, Jeroen Belleman wrote:
> The six-pointed stars we see in all JWST images are rather
> disturbing, yet, it should be easy enough to filter them out.
> Why don't they?

What's wrong with the diffraction spikes from the JWST?

Also, there should be 8, as I recall (although the horizontal ones are
pretty dim, compared to the vertical and "X" shaped ones).

--
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On Mon, 16 Oct 2023 23:24:24 -0000 (UTC), Dan Purgert <dan@djph.net>
wrote:

>On 2023-10-16, Jeroen Belleman wrote:
>> The six-pointed stars we see in all JWST images are rather
>> disturbing, yet, it should be easy enough to filter them out.
>> Why don't they?
>
>What's wrong with the diffraction spikes from the JWST?
>
>Also, there should be 8, as I recall (although the horizontal ones are
>pretty dim, compared to the vertical and "X" shaped ones).

Aren't the mirrors hexagons?

On 16/10/2023 22:49, Jeroen Belleman wrote:
> The six-pointed stars we see in all JWST images are rather
> disturbing, yet, it should be easy enough to filter them out.

It is a characteristic feature of the hexagonal aperture symmetry and
there is a minor cross line resulting from the support structure too. So
it is actually a six pointed star with two short spikes across. ie

\ /
\ /
\ | /
______\|/_______
/|\
/ | \
/ \
/ \

Point spread function of any aperture is the Fourier transform of its
shape filed with 1's. It is a consequence of using a mirror of finite
extent. You can't get rid of the diffraction pattern without inventing
measurements for the spatial frequencies that you don't have (or
severely compromising resolution).

It is actually quite a difficult inverse problem to remove diffraction
artefacts and the raw images are much better for scientific purposes. On
a good day with a trailing wind something like Maximum Entropy will get
you 3x superresolution on the brightest point sources and some noise
suppression. But it is also inclined to add ripples into any nebulae
since the positivity contraint doesn't work well there like it does
against the blackness of space.

Some images are much tougher than others to deconvolve reliably.

I don't find them offensive YMMV. It *is* what the telescope actually saw!

> Why don't they?
>
> Jeroen Belleman

Whilst deconvolution will do it so that you get a prettier coffee book
picture and depending on the method used you end up with either
resolution that depends on local signal to noise and/or missing faint
detail in the regions where the brighter spikes used to be.

JWST PSF stands out because of the unusual symmetry of its mirror.

The six pointed star is also a feature of some ground based telescopes
with round mirrors and a 3 way centre diagonal support. They have less
total power in each of the diffraction spikes as a result. The first
biggish scope I used was of this sort. i didn't much like it back then
preferring the more normal eight pointed stars of other big scopes. ie.

|
|
\ | /
______\|/______
/|\
/ | \
|
|

Some even have weird curved supports that behave a bit like an apodising
filter.

Most big scopes these days have brutal straight 4 way central mirror
support (or none if they are catadioptric and have a front optical
surface with a silvered mirror spot on axis.

ASCII art Best viewed with a fixed width font.

--
Martin Brown

On 10/17/23 00:14, john larkin wrote:
> On Mon, 16 Oct 2023 23:49:17 +0200, Jeroen Belleman
> <jeroen@nospam.please> wrote:
>
>> The six-pointed stars we see in all JWST images are rather
>> disturbing, yet, it should be easy enough to filter them out.
>> Why don't they?
>>
>> Jeroen Belleman
>
> Why disturbing?
>

I find them objectionable because they often obscure (oblude?)
nearby detail. These days, it should be relatively easy to
Fourier-transform the image, apply some window function to remove
the hexagon pattern of the mirrors and the shadows of the secondary
mirror supports, and then transform it back again.

Jeroen Belleman

On 17/10/2023 09:43, Jeroen Belleman wrote:
> On 10/17/23 00:14, john larkin wrote:
>> On Mon, 16 Oct 2023 23:49:17 +0200, Jeroen Belleman
>> <jeroen@nospam.please> wrote:
>>
>>> The six-pointed stars we see in all JWST images are rather
>>> disturbing, yet, it should be easy enough to filter them out.
>>> Why don't they?
>>>
>>> Jeroen Belleman
>>
>> Why disturbing?
>>
>
> I find them objectionable because they often obscure (oblude?)
> nearby detail. These days, it should be relatively easy to
> Fourier-transform the image, apply some window function to remove
> the hexagon pattern of the mirrors and the shadows of the secondary
> mirror supports, and then transform it back again.

That wouldn't work without losing a hell of a lot of resolution though.
It is the *missing* spatial frequency data that is damaging the image
quality. The de-facto window function already applied by the physics of
a finite aperture is the autocorrelation of the aperture shape.

Because of the various straight sides it isn't that far off being a
triangle with a peak in the middle and linear decline to zero at the
diameter of the mirror along the direction you choose.

You can't filter away the fact that you don't have any data for some
missing spatial frequencies that you were not able to measure because of
the finite extent of the mirror and its supports.

You have to invent them to make the diffraction spikes go away. That can
be done but it isn't unambiguous or simple and introduces new artefacts
that can get in the way of interpretation.

Linear methods like Weiner filters exist that work fairly well but the
non-linear ones which are more computationally expensive do better
because they naturally include the heuristic knowledge that the sky is
everywhere positive brightness and can use the empty sky being black to
rule out impossible scenarios from their reconstructed image. This isn't
a too bad introduction to what is possible at about the right level
(with some algebra and more importantly sample pictures).

https://www2.ph.ed.ac.uk/~wjh/teaching/dia/documents/reconstruction.pdf

Any fast linear method invariably produces negative ringing around the
point sources which is way more distracting than diffraction spikes.

First you have to decide what question you want to answer and then do
the image processing that best facilitates solving your problem. It is
invariably better to start from the raw data for astrophysics.

--
Martin Brown

On 2023-10-17, John Larkin wrote:
> On Mon, 16 Oct 2023 23:24:24 -0000 (UTC), Dan Purgert <dan@djph.net>
> wrote:
>
>>On 2023-10-16, Jeroen Belleman wrote:
>>> The six-pointed stars we see in all JWST images are rather
>>> disturbing, yet, it should be easy enough to filter them out.
>>> Why don't they?
>>
>>What's wrong with the diffraction spikes from the JWST?
>>
>>Also, there should be 8, as I recall (although the horizontal ones are
>>pretty dim, compared to the vertical and "X" shaped ones).
>
> Aren't the mirrors hexagons?

Yes, the primary mirrors are hexagons, but the standoffs for the
secondary mirror also generates a 6-pointed diffraction spike. The "X"
portion just overlaps.

Here's a diagram of the elements:
https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#/media/File:JWST_diffraction_spikes.svg

--
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|_|_|O| Github: https://github.com/dpurgert
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On Tue, 17 Oct 2023 10:43:58 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 10/17/23 00:14, john larkin wrote:
>> On Mon, 16 Oct 2023 23:49:17 +0200, Jeroen Belleman
>> <jeroen@nospam.please> wrote:
>>
>>> The six-pointed stars we see in all JWST images are rather
>>> disturbing, yet, it should be easy enough to filter them out.
>>> Why don't they?
>>>
>>> Jeroen Belleman
>>
>> Why disturbing?
>>
>
>I find them objectionable because they often obscure (oblude?)
>nearby detail. These days, it should be relatively easy to
>Fourier-transform the image, apply some window function to remove
>the hexagon pattern of the mirrors and the shadows of the secondary
>mirror supports, and then transform it back again.
>
>Jeroen Belleman

NASA is basically a money-burning PR operation, and probably elects to
show star patterns in pretty public pictures.

And if the bright lines saturate the detector, whatever is under them
can't be recovered.

On 17/10/2023 14:57, John Larkin wrote:
> On Tue, 17 Oct 2023 10:43:58 +0200, Jeroen Belleman
> <jeroen@nospam.please> wrote:
>
>> On 10/17/23 00:14, john larkin wrote:
>>> On Mon, 16 Oct 2023 23:49:17 +0200, Jeroen Belleman
>>> <jeroen@nospam.please> wrote:
>>>
>>>> The six-pointed stars we see in all JWST images are rather
>>>> disturbing, yet, it should be easy enough to filter them out.
>>>> Why don't they?
>>>>
>>>> Jeroen Belleman
>>>
>>> Why disturbing?
>>>
>>
>> I find them objectionable because they often obscure (oblude?)
>> nearby detail. These days, it should be relatively easy to
>> Fourier-transform the image, apply some window function to remove
>> the hexagon pattern of the mirrors and the shadows of the secondary
>> mirror supports, and then transform it back again.
>>
>> Jeroen Belleman
>
> NASA is basically a money-burning PR operation, and probably elects to
> show star patterns in pretty public pictures.

It is the other way around. The star pattern is a diffraction effect
from the physical shape of the aperture which was made up of hexagons so
that it could fold up into a tight cylindrical space for launch.

It was news to me that JWST was named after a controversial NASA manager
- I had initially thought it had been named after one of my fellow
countrymen - the Victoria amateur astronomer Rev Thomas Webb whose name
lives on in a society for the study of deep sky objects. He wrote one of
the first major work on such objects (amateur and professional astronomy
was quite blurred back then).

https://www.webbdeepsky.com/wbg/twwebb.html

Messier's name also lives on with his catalogue of 106+/-4 catalogue of
nuisance fuzzy objects when comet hunting. He tended to be drunk when
making observational notes so some of his objects are disputed. ISTR 102
of them are now fairly certain and the others are later additions.

> And if the bright lines saturate the detector, whatever is under them
> can't be recovered.

It is true that whatever was under a diffraction spike is difficult to
recover.

But you can't blame NASA for that - the laws of physics play an
important part in determining what a telescope can actually see.

There are stars that are too bright for the JWST to observe.

If anything ever saturates a detector it is actually the unresolved
point object that is at the centre of the diffraction pattern. When that
happens you get readout bleed along a column of cells.

You sometimes see it in the realtime preview of a phone being used as a
camera when there are bright lights in a dim scene. The wells in the
tiny chips are not very deep so very easily saturate.

--
Martin Brown