APOD: Webb's First Deep Field (2022 Jul 13)

Comments and questions about the APOD on the main view screen.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Thu Jul 14, 2022 1:41 pm

FLPhotoCatcher wrote: Thu Jul 14, 2022 7:12 am
Chris Peterson wrote: Thu Jul 14, 2022 5:00 am
FLPhotoCatcher wrote: Thu Jul 14, 2022 4:51 am

Why didn't they use a sensor that better matched the actual view, or at least use a bigger one, with no reduction is resolving power? That seems like a great way to get significantly more "free" data.
What makes you think the sensors don't cover all of the actual view? That they aren't matched in both pixel size and area to the optics?
As Rob said, "The answer is simple, I think. The mirrors focus light into one or more of the collectors, and they [the collectors] are rectangular."
I think the incoming light would be round-ish, and the collector is rectangle. I have no proof one way or the other though.
Yeah, that's what I would think as well. Whether JSWT uses mirrors and or lenses to focus the light onto the detectors I'm not sure. Maybe one or the other or both depending on the instrument. But I'd have to think that the image that eventually hits a detector is round, not square. And if the detector is square, either the round image fits entirely with the area of the detector or it doesn't. If it does, then no part of the image would be wasted (but some of the detector would!), but if it does not, parts of the edges of the image would be cut off. There's lots of possible answers here, but it's too technical for me:

https://jwst-docs.stsci.edu/jwst-near-i ... r-overview

This page is for NIRCam, but there are similar pages for all the other instruments.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Thu Jul 14, 2022 1:46 pm

zmon21 wrote: Thu Jul 14, 2022 9:11 am in stephan's quintet, NGC 7320 is 7 times closer than the other four galaxies, however they look almost the same size. as size is inversely proportional to the square root, the other galaxies are roughly 49 times bigger than NGC 7320.  diameter of NGC 7320 is 30,415.05 light years. then the other four galaxies diameters are around 1.5 million lys. is this right?
Angular size is inversely proportional to distance. Something twice as far away will appear half the size. But as a result, the AREA of the object will be a quarter the size.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by Keyman » Thu Jul 14, 2022 3:09 pm

Do we know why we had to wait for the release of the many images we got on the 12th, as opposed to them being released as they were 'captured'?

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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by Chris Peterson » Thu Jul 14, 2022 3:12 pm

Keyman wrote: Thu Jul 14, 2022 3:09 pm Do we know why we had to wait for the release of the many images we got on the 12th, as opposed to them being released as they were 'captured'?
They may have still been in the process of being processed. Also, it's common for there to be an embargo period on releasing data to give the PIs a window for first publishing.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Thu Jul 14, 2022 6:35 pm

Chris Peterson wrote: Thu Jul 14, 2022 3:12 pm
Keyman wrote: Thu Jul 14, 2022 3:09 pm Do we know why we had to wait for the release of the many images we got on the 12th, as opposed to them being released as they were 'captured'?
They may have still been in the process of being processed. Also, it's common for there to be an embargo period on releasing data to give the PIs a window for first publishing.
"PIs"? (that's an I not an L I take it)
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by Chris Peterson » Thu Jul 14, 2022 8:09 pm

johnnydeep wrote: Thu Jul 14, 2022 6:35 pm
Chris Peterson wrote: Thu Jul 14, 2022 3:12 pm
Keyman wrote: Thu Jul 14, 2022 3:09 pm Do we know why we had to wait for the release of the many images we got on the 12th, as opposed to them being released as they were 'captured'?
They may have still been in the process of being processed. Also, it's common for there to be an embargo period on releasing data to give the PIs a window for first publishing.
"PIs"? (that's an I not an L I take it)
Principal Investigators. Usually the one who is applying for the grant or for observation time.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Thu Jul 14, 2022 8:18 pm

Chris Peterson wrote: Thu Jul 14, 2022 8:09 pm
johnnydeep wrote: Thu Jul 14, 2022 6:35 pm
Chris Peterson wrote: Thu Jul 14, 2022 3:12 pm

They may have still been in the process of being processed. Also, it's common for there to be an embargo period on releasing data to give the PIs a window for first publishing.
"PIs"? (that's an I not an L I take it)
Principal Investigators. Usually the one who is applying for the grant or for observation time.
Thanks. I was thinking "private investigators" but suspected that couldn't be right in this context :)
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by FLPhotoCatcher » Fri Jul 15, 2022 6:34 am

johnnydeep wrote: Wed Jul 13, 2022 2:23 pm
FLPhotoCatcher wrote: Wed Jul 13, 2022 8:09 am Untitled22.JPG

I have circled a galaxy that seems to appear four times, the two with the "R" next to it seem to be mirrored.

The squared ones seem to be two of a different galaxy, also mirrored.

Can lensed galaxies be mirrored?
Why do you think that your 4 circled galaxies are the same one? They look completely unrelated to me.
They look to be the same color, and vaguely the same shape, even if mirrored. If they are, then the biggest view of it is amazingly magnified.

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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Fri Jul 15, 2022 12:47 pm

FLPhotoCatcher wrote: Fri Jul 15, 2022 6:34 am
johnnydeep wrote: Wed Jul 13, 2022 2:23 pm
FLPhotoCatcher wrote: Wed Jul 13, 2022 8:09 am Untitled22.JPG

I have circled a galaxy that seems to appear four times, the two with the "R" next to it seem to be mirrored.

The squared ones seem to be two of a different galaxy, also mirrored.

Can lensed galaxies be mirrored?
Why do you think that your 4 circled galaxies are the same one? They look completely unrelated to me.
They look to be the same color, and vaguely the same shape, even if mirrored. If they are, then the biggest view of it is amazingly magnified.
They don't appear to be of a similar shape at all to me.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by beryllium732 » Sat Jul 16, 2022 8:31 am

Karthik wrote: Wed Jul 13, 2022 10:26 am
VictorBorun wrote: Wed Jul 13, 2022 10:14 am
Karthik wrote: Wed Jul 13, 2022 8:13 am The Webb page (!) says that they are star clusters.

"One galaxy speckled with star clusters appears near the bottom end of the bright central star’s vertical diffraction
spike – just to the right of a long orange arc. The long, thin ladybug-like galaxy is flecked with pockets of star formation.
Draw a line between its “wings” to roughly match up its star clusters, mirrored top to bottom.
https://webbtelescope.org/contents/medi ... 5CSH1Q5Z1Z
Like this?bright orange dots+.jpg
They are not talking about mirroring the galaxy itself. They are saying that roughly the same number of star clusters can be seen above and below the galaxy, if we draw a line through it.
Sorry for being dumb but what do you mean? That there are equal number of star clusters below and top of the galaxy? What gives?

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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by VictorBorun » Sat Jul 16, 2022 12:35 pm

beryllium732 wrote: Sat Jul 16, 2022 8:31 am
Karthik wrote: Wed Jul 13, 2022 10:26 am
VictorBorun wrote: Wed Jul 13, 2022 10:14 am

Like this?bright orange dots+.jpg
They are not talking about mirroring the galaxy itself. They are saying that roughly the same number of star clusters can be seen above and below the galaxy, if we draw a line through it.
Sorry for being dumb but what do you mean? That there are equal number of star clusters below and top of the galaxy? What gives?
the text seems clumsy and faulty.
On another page they say that more lensed thinner galaxies beside are 2 mirror images of a distant galaxy; lines in their spectra show the same redshift and the same chemistry.
https://stsci-opo.org/STScI-01G7NGV29E6 ... MPWTVF.png
So I think the axis of mirror symmetry is something like this:
mirror.jpg
You can see 2 smaller very red galaxies, to the right and top, that I think are 2 mirror images of one distant galaxy, too.
Here I try to take the bottom-right part of the frame, flip it and compress it to make the resemblance clear
mirror 2.jpg
The same line of looking leads, in MIR
https://stsci-opo.org/STScI-01G7DDVJM97 ... 5BE3N3.png
to a few more mirror image candidate of deep red:
mirror 3.jpg
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by VictorBorun » Sat Jul 16, 2022 2:52 pm

johnnydeep wrote: Thu Jul 14, 2022 1:41 pm Whether JSWT uses mirrors and or lenses to focus the light onto the detectors I'm not sure.
It's ok to use flat "lenses" to filter the light.
It's not ok to use concave or convex lenses to focus the image, because it would destroy multi-chromatic image resolution.
A good Newtonian reflector should avoid focusing with lenses.

What is the shape of the image field focused on the sensor?
Well, it does not resemble a hexagon, criss-crossed into 18 hexagon pieces and crossed with 3 radiuses — the shadows of the 3 holders of the second mirror.
What those things do with the image is create diffractional anti-shadows — 8 spikes around every Milky Way's star and every mid-IR-quasar (those beasts has just been discovered by JWST):
Image

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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Sat Jul 16, 2022 3:11 pm

VictorBorun wrote: Sat Jul 16, 2022 2:52 pm
johnnydeep wrote: Thu Jul 14, 2022 1:41 pm Whether JSWT uses mirrors and or lenses to focus the light onto the detectors I'm not sure.
It's ok to use flat "lenses" to filter the light.
It's not ok to use concave or convex lenses to focus the image, because it would destroy multi-chromatic image resolution.
A good Newtonian reflector should avoid focusing with lenses.

What is the shape of the image field focused on the sensor?
Well, it does not resemble a hexagon, criss-crossed into 18 hexagon pieces and crossed with 3 radiuses — the shadows of the 3 holders of the second mirror.
What those things do with the image is create diffractional anti-shadows — 8 spikes around every Milky Way's star and every mid-IR-quasar (those beasts has just been discovered by JWST):
Image
So Webb doesn't use any lenses to focus light on detectors? I wish I could find a reference for that somewhere.

As for that image explaining where the diffraction spikes are coming from, that is amazingly illustrative, but I still don't really understand it. The conclusion seems to be that each set of parallel edges (either on the 18 hexagonal mirrors or from the secondary mirror support struts) end up creating diffraction spikes perpendicular to them! But why perpendicular? I'm sure there's a "obvious" explanation, which I happen to be too dense to see.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by VictorBorun » Sat Jul 16, 2022 6:18 pm

johnnydeep wrote: Sat Jul 16, 2022 3:11 pm each set of parallel edges (either on the 18 hexagonal mirrors or from the secondary mirror support struts) end up creating diffraction spikes perpendicular to them! But why perpendicular?
Consider a bright star or a bright mid-IR active core of a galaxy I nicknamed a mid-IR-quasar.
It should make a bright point in the plane of the image (=the surface of the sensor array).
And on the way to make the point it should be a smooth not-quite-flat wave front between the sensor and the mirrors and a quite flat wave front between the star and the JWST.

Now let's introduce some straight line thin and long obstruction to light where the wave front is quite flat: one of the 3 holders of the secondary mirror in the way to the primary mirror or one of the gaps between neighbouring hexagon pieces.

We involuntary are trying to block a portion of the wave, to create a shadow.
Were the wavelength of the photons we try to register infinitely small, the shadow would be a portion of space behind the obstacle. As the wavefront focuses to a point in the plane of the image, so would the shadowed portion of the wavefront. Resulting point in the plane of the image would sum up all the wavefront but the shadowed portion, and so it would be slightly less bright; it would not spoil the image.

But, alas, the wavelength does matter here. At the shadow's borders some of the wave curves into the space behind the obstacle. Every photon is a quantum that would get caught only with one pixel of the sensor, but that does not help us at all: in the flight that photon behaves like a wave and gets some chance to curve into the shadowed space behind the obstacle. So the quantum nature of the photons does not make them go along straight lines with a 100 % guarantee; statistically they create an image like waves they are.

So there are some ripples slightly getting behind the obstacle.They travel in the directions slightly different from the direction that the smooth wave front travels. The deviation is oriented perpendicular to the long thin straight obstacle. So those ripples get focused slightly wrong: not precisely into the same point as all the smooth wavefront, but with slight deviations oriented perpendicular to the long thin straight obstacle.

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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Sat Jul 16, 2022 7:34 pm

VictorBorun wrote: Sat Jul 16, 2022 6:18 pm
johnnydeep wrote: Sat Jul 16, 2022 3:11 pm each set of parallel edges (either on the 18 hexagonal mirrors or from the secondary mirror support struts) end up creating diffraction spikes perpendicular to them! But why perpendicular?
Consider a bright star or a bright mid-IR active core of a galaxy I nicknamed a mid-IR-quasar.
It should make a bright point in the plane of the image (=the surface of the sensor array).
And on the way to make the point it should be a smooth not-quite-flat wave front between the sensor and the mirrors and a quite flat wave front between the star and the JWST.

Now let's introduce some straight line thin and long obstruction to light where the wave front is quite flat: one of the 3 holders of the secondary mirror in the way to the primary mirror or one of the gaps between neighbouring hexagon pieces.

We involuntary are trying to block a portion of the wave, to create a shadow.
Were the wavelength of the photons we try to register infinitely small, the shadow would be a portion of space behind the obstacle. As the wavefront focuses to a point in the plane of the image, so would the shadowed portion of the wavefront. Resulting point in the plane of the image would sum up all the wavefront but the shadowed portion, and so it would be slightly less bright; it would not spoil the image.

But, alas, the wavelength does matter here. At the shadow's borders some of the wave curves into the space behind the obstacle. Every photon is a quantum that would get caught only with one pixel of the sensor, but that does not help us at all: in the flight that photon behaves like a wave and gets some chance to curve into the shadowed space behind the obstacle. So the quantum nature of the photons does not make them go along straight lines with a 100 % guarantee; statistically they create an image like waves they are.

So there are some ripples slightly getting behind the obstacle.They travel in the directions slightly different from the direction that the smooth wave front travels. The deviation is oriented perpendicular to the long thin straight obstacle. So those ripples get focused slightly wrong: not precisely into the same point as all the smooth wavefront, but with slight deviations oriented perpendicular to the long thin straight obstacle.
Thanks for trying, but I'm afraid your explanation is too nuanced for me.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by Chris Peterson » Sat Jul 16, 2022 10:39 pm

VictorBorun wrote: Sat Jul 16, 2022 2:52 pm
johnnydeep wrote: Thu Jul 14, 2022 1:41 pm Whether JSWT uses mirrors and or lenses to focus the light onto the detectors I'm not sure.
It's ok to use flat "lenses" to filter the light.
It's not ok to use concave or convex lenses to focus the image, because it would destroy multi-chromatic image resolution.
Not true. You can certainly design well corrected refractive optics. Many professional astronomical cameras used in optical wavelength ranges use internal refractive elements, even when they're on reflecting telescopes.
A good Newtonian reflector should avoid focusing with lenses.
Most serious imagers using Newtonians have moderately complex lens systems sitting between the mirror and the sensor to correct for coma and astigmatism, which are unavoidable even with a perfect mirror.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by Chris Peterson » Sat Jul 16, 2022 10:58 pm

johnnydeep wrote: Sat Jul 16, 2022 3:11 pm So Webb doesn't use any lenses to focus light on detectors? I wish I could find a reference for that somewhere.
The NIRCam instrument (the primary imager) has a bunch of lenses in the optical path. The light coming from the objective (consisting of three mirrors, an ellipsoidal primary, hyperboloidal secondary, and ellipsoidal tertiary) is diverted through a series of both lenses and mirrors on its way to the detectors.
As for that image explaining where the diffraction spikes are coming from, that is amazingly illustrative, but I still don't really understand it. The conclusion seems to be that each set of parallel edges (either on the 18 hexagonal mirrors or from the secondary mirror support struts) end up creating diffraction spikes perpendicular to them! But why perpendicular? I'm sure there's a "obvious" explanation, which I happen to be too dense to see.
Optics are evaluated by considering the point spread function (PSF). This is basically the image you get from a point source. A star is a pretty good one! With the simplest optics, consisting of a circular objective, your image consists of a bright central spot surrounded by rings (called the Airy disc and Airy rings). This is produced by the diffraction of light around the edge of the aperture. In the case of the JWST, you have a hexagonal aperture, not a circular one, so you don't get a disc with rings, but something that looks more like a snowflake. Every star in a JWST image will look like that if it's bright enough to occupy more than a single pixel.

Now if you place a string across the aperture, you'll produce a diffraction spike that is perpendicular to the string. I've seen the term "shadow" or "shadowing" come up here and there above. There is no shadowing involved. Diffraction is always perpendicular to edges. You might picture it as a kind of scattering. If you crash into a pole, you don't bounce along the length of the pole, you bounce to one side or the other.

The JWST aperture consists of edges made of of the hexagonal elements, which produce 12 spikes (but they are paired edges, so you get spike pairs... that is, visually only 6 spikes). And then there is a support for the secondary that isn't parallel to any of the hexagonal edges. That produces the weaker diffraction spikes we see. All adding up to a little snowflake surrounded by 8 spikes.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by alter-ego » Sun Jul 17, 2022 12:33 am

johnnydeep wrote: Sat Jul 16, 2022 3:11 pm
VictorBorun wrote: Sat Jul 16, 2022 2:52 pm
johnnydeep wrote: Thu Jul 14, 2022 1:41 pm Whether JSWT uses mirrors and or lenses to focus the light onto the detectors I'm not sure.
It's ok to use flat "lenses" to filter the light.
It's not ok to use concave or convex lenses to focus the image, because it would destroy multi-chromatic image resolution.
A good Newtonian reflector should avoid focusing with lenses.

What is the shape of the image field focused on the sensor?
Well, it does not resemble a hexagon, criss-crossed into 18 hexagon pieces and crossed with 3 radiuses — the shadows of the 3 holders of the second mirror.
What those things do with the image is create diffractional anti-shadows — 8 spikes around every Milky Way's star and every mid-IR-quasar (those beasts has just been discovered by JWST):
Image
So Webb doesn't use any lenses to focus light on detectors? I wish I could find a reference for that somewhere.

As for that image explaining where the diffraction spikes are coming from, that is amazingly illustrative, but I still don't really understand it. The conclusion seems to be that each set of parallel edges (either on the 18 hexagonal mirrors or from the secondary mirror support struts) end up creating diffraction spikes perpendicular to them! But why perpendicular? I'm sure there's a "obvious" explanation, which I happen to be too dense to see.
• Focusing light is done two ways (excluding gravitational lensing :D), and both lenses (refractive) and mirrors (reflective) manage all sorts of imaging situations. In fact, both lenses and mirrors are used in Webb. NIRCam uses sets of collimator lenses before the FPA (focal plane array). MIRI and NIRSpec use only mirrors to "focus" light for imaging. However, the MIRI LRS (low-resolution spectrometer) does introduce a transmissive dual prism (Zn and Ge substrates) to disperse the light, which is very reasonable and easier to implement for low-resolution spectrometry.

A quick search landed these articles, and I think they suffice to answer your first question: Webb employs a hybrid of reflective and refractive optical components depending on application.

• Regarding diffraction, mathematics formally answers your question. It's a bit difficult to explain if you're not familiar with the physics, and unfortunately wrt Webb, edge diffraction dominates and is a bit more complicated than single slit diffraction. However, the principles are the same. Said simply:
→ The fringe pattern below the edge is caused by interference of the edge-diffracted light and the unblocked light, and
→ The reason why the fringes are parallel to the edge is buried in the theory
Scientific Reports wrote: Among these, Fresnel’s approach based on Huygens’s principle is well known26,27. It describes the diffraction field in terms of superposition of two waves: one wave propagates through the diffracting object without any perturbation (called the geometrical wave) and the second wave originates from every point of the illuminated boundary of the object or edge (called the boundary diffracted wave).
Scientific Reports
Note, the figure ignores diffraction at the side edges (assume the block is infinitely long). For a finite long block and a light beam wider than the block, you would see identical fringes on the left and right sides of the block with a transition at each sharp corner.
Edge Diffraction - Nature.jpg

Chapter 22
Note, below is a single-slit diffraction illuminated with collimated light without a focusing lens after the slit. Inserting a lens will squish the (vertical) height of the diffraction fringes down to be diffraction-limited.
Single Slit - Diffraction of Light, Power Point.jpg
FYI, Wolfram has a single-slit simulator you can play with: Single-Slit Diffraction Simulator

Bottom Line: Diffraction fringes form along an axis perpendicular to the edge (linear or curved)

Anecdotally, light incident on a slit (Webb mirror-segment separations), and equivalently, narrow opaque edges (secondary mirror support) will spread away from the slit or edge. Because the edge lengths are much longer (>> wavelength) than it is wide, visible diffraction dominantly occurs in one axis along the narrow-width direction, i.e. perpendicular to the edges.
→ Think of a single slit illuminated by collimated light (e.g. star light). Looking at the transmitted light on a card near the slit, you'll see a wavelength dependent, periodic slit-like pattern (fringes) having diminishing brightness the further from the slit you look. The visible fringes are much longer than they are wide. Ok, that's up close. Now put a lens immediately after the slit and look at the focal plane of the lens (like a telescope) and you'll see a narrow set of fringes (i.e. not the length of the slit anymore) similarly propagating away from the slit. As stated, this occurs because diffraction imparts an angle to the transmitted light only in the direction perpendicular to the long axis of the slit.Light is negligibly diffracted along the slit's long axis because any edges are too far apart to significantly diffract the light.
Now, consider a simple refractor. The only diffracting aperture is at the entrance of the telescope. Here, light diffracts perpendicular to the circular edge (i.e. radially. So, at the telescope's prime focus, you'll similarly see periodic diffraction rings :idea: This is the Airy pattern. In reality, basic astronomical telescope designs with a circular primary mirror and secondary mirror supports reveal a diffraction pattern combining the Airy pattern with support vane diffraction.
→ The JWST is definitely more complicated, but the same principles apply.

I hope this increases your understanding of diffraction, at least up to, but excluding, the theory.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by VictorBorun » Sun Jul 17, 2022 10:16 am

isoparix wrote: Wed Jul 13, 2022 3:35 pm These images are, I suppose, false-colour representations of infrared data. How much has the spectrum been shifted upwards - two octaves? ten?
There are 2 shifts at play.

1) blueshift of near IR to RGB presentation by this legend:
https://stsci-opo.org/STScI-01G7NC1B193 ... 5H41YN.png
4.44 μm > 0.65 μm red subpixel, waves got 7 times shorter

2) cosmological redshift
https://stsci-opo.org/STScI-01G7NH26MEX ... V9VN2W.png
11.3 billion years of look-back time must be from spectral lines' wavelengths 3.6 times longer
13.1 billion years of look-back time must be from spectral lines' wavelengths 8.2 times longer
Here I used z from a calculator and then calculated z+1 which is wavelengths' ratio

3) how can we correct the image of a galaxy 13.1 billion years of look-back time?
I think we should make it (z+1) = 8.2 times brighter, make the wavelengths (z+1) = 8.2 times shorter and write a comment "a portrait of a galaxy in its youth, 13.1 billion years ago". No angle size correction needed: as the space expands, the angular size of a distant galaxy is preserved precisely as it was in the moment the light was emitted.
In some special cases there is gravitational lensing by a heavy midground galaxy cluster. Things get complicated then. Colour correction does not change much, the age of the picture does not change much, but the brightness and the angular size can get larger than life many times and the shape may get stretched and even fragmented and replicated
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by VictorBorun » Sun Jul 17, 2022 10:27 am

De58te wrote: Wed Jul 13, 2022 3:33 pm Nice. What surprised me most is that the James Webb's images from its hexagonal mirrors still produces rectangular images with perfect 90 degree corners. Unlike my binoculars that has round lenses and it produces a round image. The explanation was for the old round camera lenses producing square images was because that is how the photo negative film was made. And of course science text books were rectangular and demanded rectangular photos. But nowadays when paper books are on the way out and most images are processed by computer there is no reason to still cut the round or hexagonal images into squares.
1. I doubt the best part of the JWST's image is as hexagonal as the primary mirror. I think of its shape as a bright spot on the floor under a large and high hexagonal chandelier. No crisp edges

2. web design is young. Of course it is possible to code a software to arrange hexagonal pieces of text and images in a screen lay-out

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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by Chris Peterson » Sun Jul 17, 2022 1:28 pm

VictorBorun wrote: Sun Jul 17, 2022 10:27 am
De58te wrote: Wed Jul 13, 2022 3:33 pm Nice. What surprised me most is that the James Webb's images from its hexagonal mirrors still produces rectangular images with perfect 90 degree corners. Unlike my binoculars that has round lenses and it produces a round image. The explanation was for the old round camera lenses producing square images was because that is how the photo negative film was made. And of course science text books were rectangular and demanded rectangular photos. But nowadays when paper books are on the way out and most images are processed by computer there is no reason to still cut the round or hexagonal images into squares.
1. I doubt the best part of the JWST's image is as hexagonal as the primary mirror. I think of its shape as a bright spot on the floor under a large and high hexagonal chandelier. No crisp edges

2. web design is young. Of course it is possible to code a software to arrange hexagonal pieces of text and images in a screen lay-out
Again, the corrected area on the image plane is round, and there are very good reasons for not using a round sensor.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Sun Jul 17, 2022 1:45 pm

Thanks Chris and alter-ego! That helped, though I'll have to read your replies a few more times. But what REALLY helped me was an image from the Webb telescope site itself - https://webbtelescope.org/contents/medi ... 1GAMSHKSSN - that made it crystal clear to me (aside from the math):

https://stsci-opo.org/STScI-01G6933BG2J ... 1TCPJ9.png


The money shot for me was from this close-up:

webb diffraction spike fundamentals.JPG

So, the edges of the mirrors are causing the parallel "ripple" patterns to propagate out from the edge perpendicularly, and THAT's why the spikes are perpendicular to the edges of the mirrors! Seems painfully obvious (again, disregarding the math) to me now. For some reason I had been thinking that the "spikes" were some sort of more direct image of the edges themselves. But of course, that's dead wrong.

The pic didn't specifically show the details for how the diffraction pattern results in the secondary mirror strut spikes so I made my own:

webb secondary mirror strut diffraction pattern explanation.png

So, thanks for putting me on the right track guys!
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by Chris Peterson » Sun Jul 17, 2022 1:59 pm

johnnydeep wrote: Sun Jul 17, 2022 1:45 pm Thanks Chris and alter-ego! That helped, though I'll have to read your replies a few more times.

The pic didn't specifically show the details for how the diffraction pattern results in the secondary mirror strut spikes so I made my own:
webb secondary mirror strut diffraction pattern explanation.png
So, we don't see the spikes shown here in red and blue, because they align with hexagonal edge spikes. All they do is make those spikes just a little bit brighter. The only element that isn't parallel with anything else is the main strut, creating the spike you've shown in yellow.

The little sort of dotted line pattern on the spikes (which you've shown like fence posts) is unrelated to the orientation of the obstructions. Those are minima and maxima produced by interference, and are a consequence of the wave nature of light. They are only fully apparent in monochromatic light (so in images made with narrowband filters). Different wavelengths produce differently spaced minima and maxima, so broadband images have them all washed out, and the spikes look pretty solid. We see the same thing in HST images.
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by johnnydeep » Sun Jul 17, 2022 2:17 pm

Chris Peterson wrote: Sun Jul 17, 2022 1:59 pm
johnnydeep wrote: Sun Jul 17, 2022 1:45 pm Thanks Chris and alter-ego! That helped, though I'll have to read your replies a few more times.

The pic didn't specifically show the details for how the diffraction pattern results in the secondary mirror strut spikes so I made my own:

webb secondary mirror strut diffraction pattern explanation.png
So, we don't see the spikes shown here in red and blue, because they align with hexagonal edge spikes. All they do is make those spikes just a little bit brighter. The only element that isn't parallel with anything else is the main strut, creating the spike you've shown in yellow.

The little sort of dotted line pattern on the spikes (which you've shown like fence posts) is unrelated to the orientation of the obstructions. Those are minima and maxima produced by interference, and are a consequence of the wave nature of light. They are only fully apparent in monochromatic light (so in images made with narrowband filters). Different wavelengths produce differently spaced minima and maxima, so broadband images have them all washed out, and the spikes look pretty solid. We see the same thing in HST images.
Ok, first, thanks for the clarification about only the yellow diffraction pattern from one of the secondary mirror struts resulting in a NEW spike that doesn't align with the spikes from the mirror segment edges.

But hold on now. You said "The little sort of dotted line pattern on the spikes (which you've shown like fence posts) is unrelated to the orientation of the obstructions." Isn't it precisely the orientation of the obstruction (edges of mirror segments or support struts) that is CAUSING the PARALLEL min/max pattern of ripples to propagate out to create the spike we see? Or are you saying that it's some other effect entirely that is producing the spikes perpendicular to the obstructions that has nothing to do with the identically oriented the min/max ripples? If so, - hoo boy! - then I completely did NOT understand what was happening AT ALL!
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Re: APOD: Webb's First Deep Field (2022 Jul 13)

Post by Chris Peterson » Sun Jul 17, 2022 2:39 pm

johnnydeep wrote: Sun Jul 17, 2022 2:17 pm
Chris Peterson wrote: Sun Jul 17, 2022 1:59 pm
johnnydeep wrote: Sun Jul 17, 2022 1:45 pm Thanks Chris and alter-ego! That helped, though I'll have to read your replies a few more times.

The pic didn't specifically show the details for how the diffraction pattern results in the secondary mirror strut spikes so I made my own:

webb secondary mirror strut diffraction pattern explanation.png
So, we don't see the spikes shown here in red and blue, because they align with hexagonal edge spikes. All they do is make those spikes just a little bit brighter. The only element that isn't parallel with anything else is the main strut, creating the spike you've shown in yellow.

The little sort of dotted line pattern on the spikes (which you've shown like fence posts) is unrelated to the orientation of the obstructions. Those are minima and maxima produced by interference, and are a consequence of the wave nature of light. They are only fully apparent in monochromatic light (so in images made with narrowband filters). Different wavelengths produce differently spaced minima and maxima, so broadband images have them all washed out, and the spikes look pretty solid. We see the same thing in HST images.
Ok, first, thanks for the clarification about only the yellow diffraction pattern from one of the secondary mirror struts resulting in a NEW spike that doesn't align with the spikes from the mirror segment edges.

But hold on now. You said "The little sort of dotted line pattern on the spikes (which you've shown like fence posts) is unrelated to the orientation of the obstructions." Isn't it precisely the orientation of the obstruction (edges of mirror segments or support struts) that is CAUSING the PARALLEL min/max pattern of ripples to propagate out to create the spike we see? Or are you saying that it's some other effect entirely that is producing the spikes perpendicular to the obstructions that has nothing to do with the identically oriented the min/max ripples? If so, - hoo boy! - then I completely did NOT understand what was happening AT ALL!
The linear obstruction causes a point source to have a perpendicular spike. The minima and maxima can only lie on that spike, of course. They would look the same no matter what the shape of the obstruction, and would not be present given a white light source. (My comment was largely related to your drawing the minima and maxima as lines, suggesting an orientation relationship with the obstruction that isn't there.)
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