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tech / sci.astro.amateur / Re: Like the Hubble, before the fix

SubjectAuthor
* Re: Like the Hubble, before the fixJos Bergervoet
+- Re: Like the Hubble, before the fixQuadibloc
+- Re: Like the Hubble, before the fixQuadibloc
`* Re: Like the Hubble, before the fixMartin Brown
 `* Re: Like the Hubble, before the fixChris L Peterson
  `* Re: Like the Hubble, before the fixJos Bergervoet
   +- Re: Like the Hubble, before the fixChris L Peterson
   `* Re: Like the Hubble, before the fixMartin Brown
    +* Re: Like the Hubble, before the fixChris L Peterson
    |`* Re: Like the Hubble, before the fixMartin Brown
    | `- Re: Like the Hubble, before the fixChris L Peterson
    `- Re: Like the Hubble, before the fixChris L Peterson

1
Re: Like the Hubble, before the fix

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From: jos.berg...@xs4all.nl (Jos Bergervoet)
Date: Sun, 13 Feb 2022 12:27:25 +0100
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 by: Jos Bergervoet - Sun, 13 Feb 2022 11:27 UTC

On 22/02/13 9:31 AM, RichA wrote:
> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>

If they can see this pattern, why can't they just move them all
together now with a few instructions to the actuators? Why should
it still take several months? (Actually two of them are almost
aligned already!)

Could it be that they just want them to be *not* aligned in order
to play with the individual mirror shaping first (to turn it into
18 very sharp points?)

--
Jos

Re: Like the Hubble, before the fix

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Subject: Re: Like the Hubble, before the fix
From: jsav...@ecn.ab.ca (Quadibloc)
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 by: Quadibloc - Sun, 13 Feb 2022 15:01 UTC

On Sunday, February 13, 2022 at 4:27:29 AM UTC-7, Jos Bergervoet wrote:

> If they can see this pattern, why can't they just move them all
> together now with a few instructions to the actuators?

What amazes me is how they know which of the dots is made up of
light coming from which mirror. Have they taken 18 other pictures, each
one with just one mirror moved slightly?

Yes, once they know which dot is which, it seems they have enough
information to proceed directly towards a focused image.

However, there is more going on. One of the things is that they can only
move one mirror at a time, and that one very slowly, because moving the
mirrors creates heat... and they need the mirror to be very, very cold.

John Savard

Re: Like the Hubble, before the fix

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Subject: Re: Like the Hubble, before the fix
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 by: Quadibloc - Sun, 13 Feb 2022 15:05 UTC

On Sunday, February 13, 2022 at 4:27:29 AM UTC-7, Jos Bergervoet wrote:

> Could it be that they just want them to be *not* aligned in order
> to play with the individual mirror shaping first (to turn it into
> 18 very sharp points?)

I wasn't aware that they _could_ deform those mirrors, although if that
were true, it certainly would be a reasonable thing to do.

Also, the star they're using for it is one that will be too bright to look
at once the mirrors are aligned! But if that's so, how can they use it
to align the mirrors?

John Savard

Re: Like the Hubble, before the fix

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From: '''newsp...@nonad.co.uk (Martin Brown)
Newsgroups: sci.astro.amateur
Subject: Re: Like the Hubble, before the fix
Date: Mon, 14 Feb 2022 09:18:39 +0000
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 by: Martin Brown - Mon, 14 Feb 2022 09:18 UTC

On 13/02/2022 11:27, Jos Bergervoet wrote:
> On 22/02/13 9:31 AM, RichA wrote:
>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>
> If they can see this pattern, why can't they just move them all
> together now with a few instructions to the actuators? Why should
> it still take several months? (Actually two of them are almost
> aligned already!)

Remember they have 18 mirrors making 18 images but they don't at present
know which one is which. The next step they will move probably 8 or 12
of them slightly in distinct compass directions to ID them without
changing the pattern too much. Then they move them closer together but
not quite touching and in the same pattern as the mirrors themselves.

They can get the raw images approximately superimposed to first order.

Then comes the really difficult part of making a holograph of the
composite mirror surface and fine tuning the individual focus settings
until all the panels are exactly in phase for the paraxial ray.

I can't recall how long it took them to do it for the Jodrell Bank dish
refiguring but it was a few months.

> Could it be that they just want them to be *not* aligned in order
> to play with the individual mirror shaping first (to turn it into
> 18 very sharp points?)

Getting it perfect involves deliberately introducing some phase errors
and moving the scope through focus taking images as you go. Once they
get a firing solution they will move them all at once to the final
setting but that will take some time and a lot of computer power on the
ground to interpret the large datasets it produces.

--
Regards,
Martin Brown

Re: Like the Hubble, before the fix

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From: clp...@alumni.caltech.edu (Chris L Peterson)
Newsgroups: sci.astro.amateur
Subject: Re: Like the Hubble, before the fix
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 by: Chris L Peterson - Mon, 14 Feb 2022 15:50 UTC

On Mon, 14 Feb 2022 09:18:39 +0000, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 13/02/2022 11:27, Jos Bergervoet wrote:
>> On 22/02/13 9:31 AM, RichA wrote:
>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>
>> If they can see this pattern, why can't they just move them all
>> together now with a few instructions to the actuators? Why should
>> it still take several months? (Actually two of them are almost
>> aligned already!)
>
>Remember they have 18 mirrors making 18 images but they don't at present
>know which one is which.

They know which segment created each of the individual images.

Re: Like the Hubble, before the fix

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From: jos.berg...@xs4all.nl (Jos Bergervoet)
Date: Wed, 16 Feb 2022 12:04:40 +0100
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 by: Jos Bergervoet - Wed, 16 Feb 2022 11:04 UTC

On 22/02/14 4:50 PM, Chris L Peterson wrote:
> On Mon, 14 Feb 2022 09:18:39 +0000, Martin Brown
> <'''newspam'''@nonad.co.uk> wrote:
>
>> On 13/02/2022 11:27, Jos Bergervoet wrote:
>>> On 22/02/13 9:31 AM, RichA wrote:
>>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>>
>>> If they can see this pattern, why can't they just move them all
>>> together now with a few instructions to the actuators? Why should
>>> it still take several months? (Actually two of them are almost
>>> aligned already!)
>>
>> Remember they have 18 mirrors making 18 images but they don't at present
>> know which one is which.
>
> They know which segment created each of the individual images.
>
Yes, because there was also a version of that image with annotated
segment names. Still, I'd think that individual segment adjustment
will be done before they join the segments (I mean, first doing the
slight tweaking of the berilium frames to get sharp points and then
moving the points together.)

--
Jos

Re: Like the Hubble, before the fix

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From: clp...@alumni.caltech.edu (Chris L Peterson)
Newsgroups: sci.astro.amateur
Subject: Re: Like the Hubble, before the fix
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 by: Chris L Peterson - Wed, 16 Feb 2022 14:17 UTC

On Wed, 16 Feb 2022 12:04:40 +0100, Jos Bergervoet
<jos.bergervoet@xs4all.nl> wrote:

>On 22/02/14 4:50 PM, Chris L Peterson wrote:
>> On Mon, 14 Feb 2022 09:18:39 +0000, Martin Brown
>> <'''newspam'''@nonad.co.uk> wrote:
>>
>>> On 13/02/2022 11:27, Jos Bergervoet wrote:
>>>> On 22/02/13 9:31 AM, RichA wrote:
>>>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>>>
>>>> If they can see this pattern, why can't they just move them all
>>>> together now with a few instructions to the actuators? Why should
>>>> it still take several months? (Actually two of them are almost
>>>> aligned already!)
>>>
>>> Remember they have 18 mirrors making 18 images but they don't at present
>>> know which one is which.
>>
>> They know which segment created each of the individual images.
>>
>Yes, because there was also a version of that image with annotated
>segment names. Still, I'd think that individual segment adjustment
>will be done before they join the segments (I mean, first doing the
>slight tweaking of the berilium frames to get sharp points and then
>moving the points together.)

NASA has a video out which shows the process. First they will align
the mirrors so the individual images are distributed to spatially
match the mirror pattern. Then they will move them again so that all
the images are in the same spot. Then the integrated adaptive optics
system will take over and move the mirrors automatically to manage
wavefront control.

Re: Like the Hubble, before the fix

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From: '''newsp...@nonad.co.uk (Martin Brown)
Newsgroups: sci.astro.amateur
Subject: Re: Like the Hubble, before the fix
Date: Wed, 16 Feb 2022 15:23:51 +0000
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 by: Martin Brown - Wed, 16 Feb 2022 15:23 UTC

On 16/02/2022 11:04, Jos Bergervoet wrote:
> On 22/02/14 4:50 PM, Chris L Peterson wrote:
>> On Mon, 14 Feb 2022 09:18:39 +0000, Martin Brown
>> <'''newspam'''@nonad.co.uk> wrote:
>>
>>> On 13/02/2022 11:27, Jos Bergervoet wrote:
>>>> On 22/02/13 9:31 AM, RichA wrote:
>>>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>>>>
>>>>
>>>> If they can see this pattern, why can't they just move them all
>>>> together now with a few instructions to the actuators? Why should
>>>> it still take several months? (Actually two of them are almost
>>>> aligned already!)
>>>
>>> Remember they have 18 mirrors making 18 images but they don't at present
>>> know which one is which.
>>
>> They know which segment created each of the individual images.
>>
> Yes, because there was also a version of that image with annotated
> segment names. Still, I'd think that individual segment adjustment
> will be done before they join the segments (I mean, first doing the
> slight tweaking of the berilium frames to get sharp points and then
> moving the points together.)

I would expect them to move each image to fairly close proximity to the
centre of the field and on the same side or exact opposite side of dead
centre for the purposes of more easily solving the phase equations.

Each off axis mirror segment will only perform optimally when it is
aligned to make an image at or near the optic axis. It makes no sense to
try and sharpen them up until all the crude position adjustments are
complete.

If you want to see the sorts of diffraction patterns it will have try a
hexagonal mask on your APO or more realistic a zigzag hexagonal mask.

--
Regards,
Martin Brown

Re: Like the Hubble, before the fix

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From: clp...@alumni.caltech.edu (Chris L Peterson)
Newsgroups: sci.astro.amateur
Subject: Re: Like the Hubble, before the fix
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 by: Chris L Peterson - Wed, 16 Feb 2022 18:32 UTC

On Wed, 16 Feb 2022 15:23:51 +0000, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 16/02/2022 11:04, Jos Bergervoet wrote:
>> On 22/02/14 4:50 PM, Chris L Peterson wrote:
>>> On Mon, 14 Feb 2022 09:18:39 +0000, Martin Brown
>>> <'''newspam'''@nonad.co.uk> wrote:
>>>
>>>> On 13/02/2022 11:27, Jos Bergervoet wrote:
>>>>> On 22/02/13 9:31 AM, RichA wrote:
>>>>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>>>>>
>>>>>
>>>>> If they can see this pattern, why can't they just move them all
>>>>> together now with a few instructions to the actuators? Why should
>>>>> it still take several months? (Actually two of them are almost
>>>>> aligned already!)
>>>>
>>>> Remember they have 18 mirrors making 18 images but they don't at present
>>>> know which one is which.
>>>
>>> They know which segment created each of the individual images.
>>>
>> Yes, because there was also a version of that image with annotated
>> segment names. Still, I'd think that individual segment adjustment
>> will be done before they join the segments (I mean, first doing the
>> slight tweaking of the berilium frames to get sharp points and then
>> moving the points together.)
>
>I would expect them to move each image to fairly close proximity to the
>centre of the field and on the same side or exact opposite side of dead
>centre for the purposes of more easily solving the phase equations.

As I understand it, they don't manually deal with the phase. The
telescope does that autonomously. All they need to do is get each
mirror aligned so that they all place the stellar image at the same
point.

Re: Like the Hubble, before the fix

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From: '''newsp...@nonad.co.uk (Martin Brown)
Newsgroups: sci.astro.amateur
Subject: Re: Like the Hubble, before the fix
Date: Thu, 17 Feb 2022 11:42:43 +0000
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 by: Martin Brown - Thu, 17 Feb 2022 11:42 UTC

On 16/02/2022 18:32, Chris L Peterson wrote:
> On Wed, 16 Feb 2022 15:23:51 +0000, Martin Brown
> <'''newspam'''@nonad.co.uk> wrote:
>
>> On 16/02/2022 11:04, Jos Bergervoet wrote:
>>> On 22/02/14 4:50 PM, Chris L Peterson wrote:
>>>> On Mon, 14 Feb 2022 09:18:39 +0000, Martin Brown
>>>> <'''newspam'''@nonad.co.uk> wrote:
>>>>
>>>>> On 13/02/2022 11:27, Jos Bergervoet wrote:
>>>>>> On 22/02/13 9:31 AM, RichA wrote:
>>>>>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>>>>>>
>>>>>>
>>>>>> If they can see this pattern, why can't they just move them all
>>>>>> together now with a few instructions to the actuators? Why should
>>>>>> it still take several months? (Actually two of them are almost
>>>>>> aligned already!)
>>>>>
>>>>> Remember they have 18 mirrors making 18 images but they don't at present
>>>>> know which one is which.
>>>>
>>>> They know which segment created each of the individual images.
>>>>
>>> Yes, because there was also a version of that image with annotated
>>> segment names. Still, I'd think that individual segment adjustment
>>> will be done before they join the segments (I mean, first doing the
>>> slight tweaking of the berilium frames to get sharp points and then
>>> moving the points together.)
>>
>> I would expect them to move each image to fairly close proximity to the
>> centre of the field and on the same side or exact opposite side of dead
>> centre for the purposes of more easily solving the phase equations.
>
> As I understand it, they don't manually deal with the phase. The
> telescope does that autonomously. All they need to do is get each
> mirror aligned so that they all place the stellar image at the same
> point.

They may not do it manually any more but it is definitely using the same
holographic dOTF phase retrieval algorithm as Jodrell Bank for segments.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4814230/

I would be a little surprised if the on board computer had enough
compute power to do this sort of analysis in real time.

--
Regards,
Martin Brown

Re: Like the Hubble, before the fix

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From: clp...@alumni.caltech.edu (Chris L Peterson)
Newsgroups: sci.astro.amateur
Subject: Re: Like the Hubble, before the fix
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References: <30f811bf-36ee-4670-a818-70a258c12148n@googlegroups.com> <nnd$335ff004$296e91b6@3a3209fc79436ab5> <sud6pf$k1s$1@gioia.aioe.org> <aguk0hdu23tc4jkol61ro73hlg62gt8bl5@4ax.com> <nnd$5b79f30a$62a00327@87efaa2c9b045e1a> <suj4u8$1dp5$2@gioia.aioe.org> <dogq0hpnij52csvo2prpmnasfic7jvsrr1@4ax.com> <sulcbr$j0d$1@gioia.aioe.org>
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 by: Chris L Peterson - Thu, 17 Feb 2022 14:23 UTC

On Thu, 17 Feb 2022 11:42:43 +0000, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 16/02/2022 18:32, Chris L Peterson wrote:
>> On Wed, 16 Feb 2022 15:23:51 +0000, Martin Brown
>> <'''newspam'''@nonad.co.uk> wrote:
>>
>>> On 16/02/2022 11:04, Jos Bergervoet wrote:
>>>> On 22/02/14 4:50 PM, Chris L Peterson wrote:
>>>>> On Mon, 14 Feb 2022 09:18:39 +0000, Martin Brown
>>>>> <'''newspam'''@nonad.co.uk> wrote:
>>>>>
>>>>>> On 13/02/2022 11:27, Jos Bergervoet wrote:
>>>>>>> On 22/02/13 9:31 AM, RichA wrote:
>>>>>>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>>>>>>>
>>>>>>>
>>>>>>> If they can see this pattern, why can't they just move them all
>>>>>>> together now with a few instructions to the actuators? Why should
>>>>>>> it still take several months? (Actually two of them are almost
>>>>>>> aligned already!)
>>>>>>
>>>>>> Remember they have 18 mirrors making 18 images but they don't at present
>>>>>> know which one is which.
>>>>>
>>>>> They know which segment created each of the individual images.
>>>>>
>>>> Yes, because there was also a version of that image with annotated
>>>> segment names. Still, I'd think that individual segment adjustment
>>>> will be done before they join the segments (I mean, first doing the
>>>> slight tweaking of the berilium frames to get sharp points and then
>>>> moving the points together.)
>>>
>>> I would expect them to move each image to fairly close proximity to the
>>> centre of the field and on the same side or exact opposite side of dead
>>> centre for the purposes of more easily solving the phase equations.
>>
>> As I understand it, they don't manually deal with the phase. The
>> telescope does that autonomously. All they need to do is get each
>> mirror aligned so that they all place the stellar image at the same
>> point.
>
>They may not do it manually any more but it is definitely using the same
>holographic dOTF phase retrieval algorithm as Jodrell Bank for segments.
>
>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4814230/
>
>I would be a little surprised if the on board computer had enough
>compute power to do this sort of analysis in real time.

Why? AO systems used in dozens of telescopes manage the wavefront
analysis and mirror control in realtime, without extraordinary
computers. The system has to be realtime, or it won't work.

The JWST mirror control system is a local closed loop.

Re: Like the Hubble, before the fix

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From: clp...@alumni.caltech.edu (Chris L Peterson)
Newsgroups: sci.astro.amateur
Subject: Re: Like the Hubble, before the fix
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References: <30f811bf-36ee-4670-a818-70a258c12148n@googlegroups.com> <nnd$335ff004$296e91b6@3a3209fc79436ab5> <sud6pf$k1s$1@gioia.aioe.org> <aguk0hdu23tc4jkol61ro73hlg62gt8bl5@4ax.com> <nnd$5b79f30a$62a00327@87efaa2c9b045e1a> <suj4u8$1dp5$2@gioia.aioe.org>
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 by: Chris L Peterson - Sun, 20 Feb 2022 15:27 UTC

On Wed, 16 Feb 2022 15:23:51 +0000, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 16/02/2022 11:04, Jos Bergervoet wrote:
>> On 22/02/14 4:50 PM, Chris L Peterson wrote:
>>> On Mon, 14 Feb 2022 09:18:39 +0000, Martin Brown
>>> <'''newspam'''@nonad.co.uk> wrote:
>>>
>>>> On 13/02/2022 11:27, Jos Bergervoet wrote:
>>>>> On 22/02/13 9:31 AM, RichA wrote:
>>>>>> https://www.dpreview.com/news/7026993143/james-webb-space-telescope-sees-its-first-star-using-all-18-of-its-primary-mirror-segments
>>>>>>
>>>>>
>>>>> If they can see this pattern, why can't they just move them all
>>>>> together now with a few instructions to the actuators? Why should
>>>>> it still take several months? (Actually two of them are almost
>>>>> aligned already!)
>>>>
>>>> Remember they have 18 mirrors making 18 images but they don't at present
>>>> know which one is which.
>>>
>>> They know which segment created each of the individual images.
>>>
>> Yes, because there was also a version of that image with annotated
>> segment names. Still, I'd think that individual segment adjustment
>> will be done before they join the segments (I mean, first doing the
>> slight tweaking of the berilium frames to get sharp points and then
>> moving the points together.)
>
>I would expect them to move each image to fairly close proximity to the
>centre of the field and on the same side or exact opposite side of dead
>centre for the purposes of more easily solving the phase equations.

They have now adjusted all of the mirrors in order to bring each image
into a pattern on the sensor that corresponds to the spatial pattern
of the mirrors themselves. Next they'll adjust them so they overlap,
which will be the end of the alignment process.

https://blogs.nasa.gov/webb/2022/02/18/webb-team-brings-18-dots-of-starlight-into-hexagonal-formation/

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