On 13/02/2022 08:31, RichA wrote:
> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>
Nothing like it at all. Hubble was a single mirror made incorrectly.
This one is so far performing exactly as intended.

If I was going to worry about anything it would be the scratch like
curved linear artefacts towards the bottom of the frame.

Incidentally the cluster of 5 images at bottom left is replicated as a
faint ghost to top left but curiously the bright pair at bottom centre
is not (even though it would appear to be in frame). ASCII art:

' .
.
' ..[missing in ghost]

'
'

The bit that looks like Orion with his belt collapsed to a single star.

This one is a multiple segment mirror with the segments not yet aligned.
It was designed that way so it isn't a surprise that the first light
image looks like 18 separate star images scattered around on the focal
plane. A pair are close together but the rest will need tweaking coarse
adjust first and then final focussing. Some are more nearly in focus
than others but they will only function correctly when they form a
coherent mirror surface with a single on axis focus and in phase.

The hexagonal symmetry is very obvious in the diffraction spikes.

--
Regards,
Martin Brown

On 22/02/13 12:35 PM, Martin Brown wrote:
> On 13/02/2022 08:31, RichA wrote:
>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
...
> The hexagonal symmetry is very obvious in the diffraction spikes.

The spikes should to a large extent cancel once the 18 images are
added coherently, I'd suppose.. Or isn't that expected?

--
Jos

On 13/02/2022 14:22, Jos Bergervoet wrote:
> On 22/02/13 12:35 PM, Martin Brown wrote:
>> On 13/02/2022 08:31, RichA wrote:
>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>
>    ...
>> The hexagonal symmetry is very obvious in the diffraction spikes.
>
> The spikes should to a large extent cancel once the 18 images are
> added coherently, I'd suppose.. Or isn't that expected?

No. The hard hexagonal edges and the grid between mirrors will always
result in a hexagonal spiked cross in the star image. It doesn't really
detract. Light grasp is way more important for seeing deep.
_
/ \ <<FT>> _\/_
\_/ /\

Our Fourier optics course one of the exam questions included the
diffraction patterns of capital letter apertures and the final answer:

NOW YOU HAVE IT

Someone who should have known better got "NOW YOU HVAE IT"!

O I Y T are fairly easy
E H N W are intermediate
A V are hard

--
Regards,
Martin Brown

On 22/02/13 5:09 PM, Martin Brown wrote:
> On 13/02/2022 14:22, Jos Bergervoet wrote:
>> On 22/02/13 12:35 PM, Martin Brown wrote:
>>> On 13/02/2022 08:31, RichA wrote:
>>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>
>>
>>     ...
>>> The hexagonal symmetry is very obvious in the diffraction spikes.
>>
>> The spikes should to a large extent cancel once the 18 images are
>> added coherently, I'd suppose.. Or isn't that expected?
>
> No. The hard hexagonal edges and the grid between mirrors will always
> result in a hexagonal spiked cross in the star image.

That some effet remains doesn't contradict that those spikes will "to a
large extent" cancel, if the 18 images are added coherently. Assuming
the gaps between the mirrors are much narrower than the diameter of the
individual elements, I would expect that ratio also to be the reduction
factor for the spikes, approximately..

> It doesn't really
> detract.

Why do you believe so? In a Fourier analysis with narrow gaps in the
input function you get a nice reduction factor as described above (of
course smoothly approaching zero if the gaps are narrowed to nothing).

Do you think they already rounded the edges to such an extend that
only higher order effects remain visible to begin with? (That might
make the simple Fourier picture less predictive, I guess..)

--
Jos