Astronomy and Space

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monkey
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Re: Astronomy and Space

Post by monkey » Fri Mar 18, 2022 5:51 am

Grumble wrote:
Thu Mar 17, 2022 11:10 pm
monkey wrote:
Thu Mar 17, 2022 11:02 pm
Gfamily wrote:
It's not clear, but as far as I can interpret from some of the descriptions, it's as though there are 4 dimensions to the alignment.

This image represents the basic alignment in 3D, which gives a good image on a flat (or gently curved) image plane.

However, AIUI the next phase is to perform 20 iterations of waveform adjustment, which involves tweaking pairs of mirrors at a time to get an even better alignment (I assume) for the other instruments.

The trouble is, I don't know what they mean. It's like knowing a phrase in a foreign language without really understanding it. I think I have a rough idea, but can't be sure.

If someone can provide a good translation, I'd be grateful.
I'll have a go!

They got all the mirrors pointing the right way using the selfie. This gives you a pretty good image, but it's not aberration* free, so it could be better. Aberrations cause the point spread function to be less than perfect, or if you like simple terms, make your focused spot go all smeary. This lowers image resolution and brightness.

The aberrations come from the wavefronts** from each segment arriving at the focus with different phases. To adjust the phase from each segment, they move them in and out very small amounts (because wavelength is small) till it's correct. e.g. If the light from one segment is arriving at the focus with advanced phase (too soon), you move the mirror outwards, so it gets there a little later and in phase. Positioning is done on a subwavelegnth scale. This is similar to adaptive optics, what you sometimes get in ground based telescopes***. The difference being on the ground they can do a wavefront measurement and correction on the fly. JWT can't do this measurement, so they use a complicated algorithm to optimise the image ). They will only have to do this once (if everything stays still), unlike the ground based AO, where the atmosphere keeps wobbling about and messing things up and you have to correct continuously. This is the step they have just completed.

As I said in another post, they have only corrected for the centre of the field of view (for one instrument). They will have a very nice image in this one spot, but as some aberations are dependent on the angle away from the optical axis, it might not be good near the edges, for example. The next step is to optimise the whole field, again by moving the mirrors in and out, but now they will sacrifice some of the image quality at the centre to improve it over the whole field. Part of this is making sure it's good in all the instruments, I believe they have an algorithm to decide what the best mirror position is for this. Like the field correction, they may sacrifice quality in one instrument, to improve in another.


*Aberrations are stuff like defocus and astigmatism, like what glasses correct, but much they go into higher orders than that, they are normally characterised by the Zernike polynomials - wiki clicky.

** The wavefront is an imaginary line that joins up all the points of a wave that have the same phase.

***And eye imaging, microscopy, and high powered lasers - wiki clicky.
Thanks monkey. So to give the absolute lies to children version that numpties like me can understand, it may be worth sacrificing some of the ultimate sharpness at the central point in order to get a better overall image (and the best average for all the instruments)?
Yes.

There's always compromises with optics. You can have an awesome image in one case, but you normally have to sacrifice somewhere else. The compromises you make depend on what you want to do and how complicated you are willing to make things*.


*Putting a telescope in space is an example of making things complicated.

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Gfamily
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Re: Astronomy and Space

Post by Gfamily » Fri Mar 18, 2022 10:48 am

This NASA blogpost tells us some more about what is involved in achieving each level of alignment

https://blogs.nasa.gov/webb/2022/02/03/ ... telescope/

The image yesterday marks the completion of step 5 fine phasing, though this is something that will be a regular event through Webb's operations.
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Re: Astronomy and Space

Post by Gfamily » Tue Apr 12, 2022 6:12 pm

400 years, we've been waiting - 400 years since the last observed Supernova in our own Galaxy.

But the galaxy NGC 5605 (in the constellation Virgo) has three Supernovae at the same time.
NGC 5605.jpg
NGC 5605.jpg (70.76 KiB) Viewed 559 times
However, it has to be noted that if you were IN that galaxy, you'd need to be in a particular spot* to see them all at the same time. Galaxies are so big that light can take tens of thousands of years to get from one side to the other, so for most beings within NGC 5605, the light from one would have faded before the light from the next would reach it.
Thinking about "things that happen at the same time" have a wibbly wobbly timey wimey uncertainty about them when you're looking at things the size of a galaxy.


*more accurately on a particular locus (we're on an extension of that locus)
My avatar was a scientific result that was later found to be 'mistaken' - I rarely claim to be 100% correct
ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn
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Re: Astronomy and Space

Post by Pishwish » Wed Apr 13, 2022 12:06 am

Anyone interested in the spaceflight industry is aware that dozens of organizations are trying to build rockets, chasing a small market that only has room for a few players. One company I hadn't heard of seems a bit more amateurish than most.

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Re: Astronomy and Space

Post by Grumble » Wed Apr 13, 2022 5:30 am

Pishwish wrote:
Wed Apr 13, 2022 12:06 am
Anyone interested in the spaceflight industry is aware that dozens of organizations are trying to build rockets, chasing a small market that only has room for a few players. One company I hadn't heard of seems a bit more amateurish than most.
Sounds like a referral to OHSA is in order.
Would you like your chakras realigned?

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jimbob
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Re: Astronomy and Space

Post by jimbob » Wed Apr 13, 2022 11:26 am

Grumble wrote:
Wed Apr 13, 2022 5:30 am
Pishwish wrote:
Wed Apr 13, 2022 12:06 am
Anyone interested in the spaceflight industry is aware that dozens of organizations are trying to build rockets, chasing a small market that only has room for a few players. One company I hadn't heard of seems a bit more amateurish than most.
Sounds like a referral to OHSA is in order.
Yup

Still, I guess it's not rocket science*
"You have to work hard, but you do not have to be very smart," Tina Sjögren added.

*yes I have seen the story.
Have you considered stupidity as an explanation

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basementer
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Re: Astronomy and Space

Post by basementer » Wed Apr 13, 2022 12:38 pm

jimbob wrote:
Wed Apr 13, 2022 11:26 am
Grumble wrote:
Wed Apr 13, 2022 5:30 am
Pishwish wrote:
Wed Apr 13, 2022 12:06 am
Anyone interested in the spaceflight industry is aware that dozens of organizations are trying to build rockets, chasing a small market that only has room for a few players. One company I hadn't heard of seems a bit more amateurish than most.
Sounds like a referral to OHSA is in order.
Yup

Still, I guess it's not rocket science*
"You have to work hard, but you do not have to be very smart," Tina Sjögren added.

*yes I have seen the story.
J D Clark's memoir "Ignition!" is a funny and terrifying read. It's about his work with rocket propellants, which are, of necessity, dangerously reactive. The people in that video have probably not got a f.cking clue copy of his book.
Money is just a substitute for luck anyway. - Tom Siddell

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Re: Astronomy and Space

Post by jimbob » Wed Apr 13, 2022 12:49 pm

basementer wrote:
Wed Apr 13, 2022 12:38 pm
jimbob wrote:
Wed Apr 13, 2022 11:26 am
Grumble wrote:
Wed Apr 13, 2022 5:30 am


Sounds like a referral to OHSA is in order.
Yup

Still, I guess it's not rocket science*
"You have to work hard, but you do not have to be very smart," Tina Sjögren added.

*yes I have seen the story.
J D Clark's memoir "Ignition!" is a funny and terrifying read. It's about his work with rocket propellants, which are, of necessity, dangerously reactive. The people in that video have probably not got a f.cking clue copy of his book.
Also some of the compounds in this

https://www.science.org/topic/blog-cate ... -work-with
Have you considered stupidity as an explanation

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Re: Astronomy and Space

Post by Pishwish » Sun Apr 17, 2022 1:01 pm

The response is an example of how not to take criticism! https://www.pythom.space/updates/pytho ... ump-update
....The two of us, Pythom founders and authors of this entry, have done four unguided expeditions to Everest, three unguided and unsupported full-length expeditions to the South- and North Poles, and sailed across the Atlantic from Europe to South America. During our expeditions, we lost many friends to the elements, and Pythom's bird logo, a chough, is to remind us of those brave women and men. This is the only bird that flies high up on Everest, all of the way to the summit. Old mountaineers say they are the souls of dead climbers.

We didn't survive all our expeditions by luck only, but by rigorous risk preparedness.....
(Oh and just to agree that there are few thing more entertaining in science writing than Derek Lowe's "Things I won't work with" columns.)

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bjn
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Re: Astronomy and Space

Post by bjn » Sun Apr 17, 2022 8:09 pm

Pishwish wrote:
Sun Apr 17, 2022 1:01 pm
The response is an example of how not to take criticism! https://www.pythom.space/updates/pytho ... ump-update
....The two of us, Pythom founders and authors of this entry, have done four unguided expeditions to Everest, three unguided and unsupported full-length expeditions to the South- and North Poles, and sailed across the Atlantic from Europe to South America. During our expeditions, we lost many friends to the elements, and Pythom's bird logo, a chough, is to remind us of those brave women and men. This is the only bird that flies high up on Everest, all of the way to the summit. Old mountaineers say they are the souls of dead climbers.

We didn't survive all our expeditions by luck only, but by rigorous risk preparedness.....
(Oh and just to agree that there are few thing more entertaining in science writing than Derek Lowe's "Things I won't work with" columns.)
FOOF!

(Thanks for having me read that particular column again.)

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Re: Astronomy and Space

Post by Gfamily » Sun Apr 17, 2022 8:28 pm

Pishwish wrote:
Sun Apr 17, 2022 1:01 pm
The response is an example of how not to take criticism! https://www.pythom.space/updates/pytho ... ump-update
....The two of us, Pythom founders and authors of this entry, have done four unguided expeditions to Everest, three unguided and unsupported full-length expeditions to the South- and North Poles, and sailed across the Atlantic from Europe to South America. During our expeditions, we lost many friends to the elements, and Pythom's bird logo, a chough, is to remind us of those brave women and men. This is the only bird that flies high up on Everest, all of the way to the summit. Old mountaineers say they are the souls of dead climbers.

We didn't survive all our expeditions by luck only, but by rigorous risk preparedness.....
Am I right that they admit to having lost 'many friends' on their expeditions, and they still think they are fit people to assess risk preparedness?
As they maybe meant their logo to signify: "Chuffin' hell!"
My avatar was a scientific result that was later found to be 'mistaken' - I rarely claim to be 100% correct
ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn
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Re: Astronomy and Space

Post by Pishwish » Mon Apr 18, 2022 12:46 am

That's how I read it.

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Re: Astronomy and Space

Post by IvanV » Fri Apr 22, 2022 10:25 am

Strong hints of NASA moving in the direction of a Uranus mission. If you are still around in 25 years to see the results.

The National Academy of Sciences has published their Decadal Strategy for Planetary Sciences and Astrobiology, setting out a mission to Uranus as the priority. In the past, such a prioritisation has tended to lead to a NASA mission to do that.

Apparently the planets are suitably arranged for a convenient take off around 2031-32, which would arrive at Uranus around 2044-45.

Having experienced so much wonder from the Mars, Jupiter and Saturn missions, especially the moons of the latter two, I've strongly hoped that there would be a mission to Uranus sending back results in my lifetime. I hope to live that long.

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Re: Astronomy and Space

Post by Grumble » Fri Apr 22, 2022 11:01 am

For a planet as far out as Uranus would it ever make sense to use the Sun for a slingshot?
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Gfamily
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Re: Astronomy and Space

Post by Gfamily » Fri Apr 22, 2022 11:05 am

Grumble wrote:
Fri Apr 22, 2022 11:01 am
For a planet as far out as Uranus would it ever make sense to use the Sun for a slingshot?
Slingshots work if passing planets because they make use of the fact that the planets have their own speed around the Sun and the slingshot allows some of that momentum to be picked up by the probe.

Going around the Sun doesn't help as there's effectively no way to pick up momentum.
My avatar was a scientific result that was later found to be 'mistaken' - I rarely claim to be 100% correct
ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn
Meta? I'd say so!

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Grumble
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Re: Astronomy and Space

Post by Grumble » Fri Apr 22, 2022 11:10 am

Gfamily wrote:
Fri Apr 22, 2022 11:05 am
Grumble wrote:
Fri Apr 22, 2022 11:01 am
For a planet as far out as Uranus would it ever make sense to use the Sun for a slingshot?
Slingshots work if passing planets because they make use of the fact that the planets have their own speed around the Sun and the slingshot allows some of that momentum to be picked up by the probe.

Going around the Sun doesn't help as there's effectively no way to pick up momentum.
Ah, so the momentum you gain falling down the well is lost as you climb back out?
Would you like your chakras realigned?

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Gfamily
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Re: Astronomy and Space

Post by Gfamily » Fri Apr 22, 2022 11:25 am

Grumble wrote:
Fri Apr 22, 2022 11:10 am
Gfamily wrote:
Fri Apr 22, 2022 11:05 am
Grumble wrote:
Fri Apr 22, 2022 11:01 am
For a planet as far out as Uranus would it ever make sense to use the Sun for a slingshot?
Slingshots work if passing planets because they make use of the fact that the planets have their own speed around the Sun and the slingshot allows some of that momentum to be picked up by the probe.

Going around the Sun doesn't help as there's effectively no way to pick up momentum.
Ah, so the momentum you gain falling down the well is lost as you climb back out?
For the Sun, yes - there's no asymmetry that allows you to pick up speed.
My avatar was a scientific result that was later found to be 'mistaken' - I rarely claim to be 100% correct
ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn
Meta? I'd say so!

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shpalman
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Re: Astronomy and Space

Post by shpalman » Fri Apr 22, 2022 11:28 am

A slingshot around a planet can be thought of as elastically bouncing off the planet to pick up extra speed (which slows the planet down by an infinitesimal amount for conservation of energy and momentum).
molto tricky

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bjn
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Re: Astronomy and Space

Post by bjn » Sat Apr 30, 2022 11:11 pm

shpalman wrote:
Fri Apr 22, 2022 11:28 am
A slingshot around a planet can be thought of as elastically bouncing off the planet to pick up extra speed (which slows the planet down by an infinitesimal amount for conservation of energy and momentum).
I get the idea that the space craft is stealing orbital kinetic energy from the planet, I just can’t visualise the exchange mechanism.

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bjn
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Re: Astronomy and Space

Post by bjn » Sat Apr 30, 2022 11:22 pm

Wikipedia to the rescue. With nice visualisations. It’s all frames of reference innit. From the point of view of the planet, the spacecraft enters and leaves with the same kinetic energy. From the point of view of the Sun they swap KE, how much and in what direction the momentum changes depends on the angle the space craft approaches the planet with respect to its orbit.

Planet point of view vs star’s point of view gif animation.
Image

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basementer
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Re: Astronomy and Space

Post by basementer » Sun May 01, 2022 4:50 am

The planet doesn't notice anything has happened, the Sun thinks the spacecraft has picked the planet's pocket and the spacecraft thinks "so long, suckers!"
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Re: Astronomy and Space

Post by tenchboy » Sun May 01, 2022 6:34 am

Ten year old you is running down the High Street on your way to meet Dobber & Mitch in West St.
There's a scaffold pole on the corner, you put your arm out and swing round the pole and -your arm tying you to the pole representing the gravitational pull of the planet - you swing round, changing direction very quickly with no noticeable loss of speed.
BRILLIAAANT!!!
For a gravitational slingshot, that scaffold pole is moving at 100 m.p.h. - its movement representing the motion of the planet around the sun - you still swing round and change direction very quickly, but you are now moving considerable faster than you were before.
Is, if I've remembered all the elements, how it was explained to me.

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Re: Astronomy and Space

Post by dyqik » Thu May 12, 2022 1:41 pm

New EHT results on the center of the Milky Way

https://beta.nsf.gov/blackholes

Image

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Re: Astronomy and Space

Post by lpm » Thu May 12, 2022 2:29 pm

It's not as big as I expected, etc etc etc
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dyqik
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Re: Astronomy and Space

Post by dyqik » Thu May 12, 2022 2:32 pm

lpm wrote:
Thu May 12, 2022 2:29 pm
It's not as big as I expected, etc etc etc
Come on now, it looks as big as a donut on the moon.

Mmmm, donuts.

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