On Wednesday, March 16, 2022 at 7:47:52 PM UTC-7, RichA wrote:
> https://phys.org/news/2022-03-space-telescope-image-star-photobombed.html
>
> Not sure.

8th mag star can show up in my binocular, I think?

On 17/03/2022 03:08, StarDust wrote:
> On Wednesday, March 16, 2022 at 7:47:52 PM UTC-7, RichA wrote:
>> https://phys.org/news/2022-03-space-telescope-image-star-photobombed.html
>>
>> Not sure.
>
> 8th mag star can show up in my binocular, I think?

Only if you have far infrared vision. The original image is online and
downloadable from Flickr by following the links from

https://www.nasa.gov/content/goddard/webb-telescope-image-galleries-from-nasa/

Direct link (which might change)

https://www.flickr.com/photos/nasawebbtelescope/51942047253/

Download at original resolution to see a lot more detail than the poxy
versions in the press. Native image size is 5437 x 3438 (odd size!).

Focus and phase on the left hand and particularly top side is spot on.

Lower half and right side there are stars with phase cancellation
nulling out their central core which should be in phase (or some other
weird instrumental artefact). I reckon the sharp edged pixelated black
pixels in bright objects are definitely an instrumental artefact.

If not then there are some very peculiar objects out there!

You can see one of each sort close together by following the diffraction
spike down from the main star pretty much a star and an anti-star! There
is also a bulls eye type artefact near a galaxy as 4 O'clock from the
antistar.

Things are definitely a bit ropey at the lower right and right hand side
with point sources trailing an assymetric horizontal diffraction spike
and phase nulled cores or some other weird sensor misbehaviour.

There appear to be hard black pixels where it should be a bright stellar
core. It is odd because the fainter stars seem to be mostly OK in focus
and the main bright star in the centre show no signs of overflow.

The number of background galaxies with exciting shapes in the background
is quite staggering. There is a rather beautiful face on spiral half on
at the bottom left with a companion almost an M51 look alike!

--
Regards,
Martin Brown

On 17/03/2022 10:18, Martin Brown wrote:
> On 17/03/2022 03:08, StarDust wrote:
>> On Wednesday, March 16, 2022 at 7:47:52 PM UTC-7, RichA wrote:
>>> https://phys.org/news/2022-03-space-telescope-image-star-photobombed.html
>>>
>>>
>>> Not sure.
>>
>> 8th mag star can show up in my binocular, I think?
>
> Only if you have far infrared vision. The original image is online and
> downloadable from Flickr by following the links from
>
> https://www.nasa.gov/content/goddard/webb-telescope-image-galleries-from-nasa/
>
>
> Direct link (which might change)
>
> https://www.flickr.com/photos/nasawebbtelescope/51942047253/
>
> Download at original resolution to see a lot more detail than the poxy
> versions in the press. Native image size is 5437 x 3438 (odd size!).
>
> Focus and phase on the left hand and particularly top side is spot on.
>
> Lower half and right side there are stars with phase cancellation
> nulling out their central core which should be in phase (or some other
> weird instrumental artefact). I reckon the sharp edged pixelated black
> pixels in bright objects are definitely an instrumental artefact.

It looks like some sort of filter glint correction algorithm that is
still a bit cack handed about how well it removes them.

If you take the image turn it into greyscale and then wind the wick up
on the dark areas by setting gamma to 3 or 4 you will see the residuals
for the rectangular grid of ghost reflected images of the main star
(much more obvious on left side). And also rectangular artefacts caused
by the sharp edges of the correction PSF.

It still works in the colour version but red background looks ugly.

> The number of background galaxies with exciting shapes in the background
> is quite staggering. There is a rather beautiful face on spiral half on
> at the bottom left with a companion almost an M51 look alike!

Way more small faint galaxies are visible (along with a few hot pixels).

--
Regards,
Martin Brown

On 21/03/2022 01:08, RichA wrote:
> On Sunday, 20 March 2022 at 09:29:57 UTC-4, Martin Brown wrote:
>> On 17/03/2022 10:18, Martin Brown wrote:

>>> Focus and phase on the left hand and particularly top side is spot on.
>>>
>>> Lower half and right side there are stars with phase cancellation
>>> nulling out their central core which should be in phase (or some other
>>> weird instrumental artefact). I reckon the sharp edged pixelated black
>>> pixels in bright objects are definitely an instrumental artefact.
>> It looks like some sort of filter glint correction algorithm that is
>> still a bit cack handed about how well it removes them.
>>
>> If you take the image turn it into greyscale and then wind the wick up
>> on the dark areas by setting gamma to 3 or 4 you will see the residuals
>> for the rectangular grid of ghost reflected images of the main star
>> (much more obvious on left side). And also rectangular artefacts caused
>> by the sharp edges of the correction PSF.
>>
[snip]
> That's interesting. I'd have expected the sensors used to be perfect. They used to charged something like 10x as much for
> that grade of sensor (CCD) over industrial or "consumer" grades. Texas Instruments once did anyway.

The sensor probably is perfect (as near as damn it). The problem is with
parallel optical filter surfaces and a very bright star in the field.

It isn't unlike the problem you sometimes get with looking at Venus
through a scope and seeing a reflection of the thing off your eyeball
and one other air glass surface.

Anti-reflection coatings are never perfect for broadband radiation.

It will only affect the brightest stars although it may require some
correction in marginal cases. I thought at first they had cleaned it up
by using a no-stars algorithm on the other field stars but the grid of
removed stars was way too regular for that to be the case.

However I found this nice comparison image from DECaLS 4m ground based
telescope which shows just how fine an image the Webb has made.

https://twitter.com/GJDonatiello/status/1504394237844115456

DECaLS has a lot more trouble with it saturating the detector too!

--
Regards,
Martin Brown