Astronomy and Space

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bjn
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Re: Astronomy and Space

Post by bjn » Fri Dec 03, 2021 9:50 pm

Somewhere or other I heard that Musks’ plan to colonise Mars will ultimately require a million tonnes of stuff be landed there.

That is a lot of launches.

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Gfamily
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Re: Astronomy and Space

Post by Gfamily » Sat Dec 11, 2021 8:36 pm

This Monday at 7pm
This might be of interest - Organised by Edinburgh Observatory, it's a chance to hear from one of the Engineers involved with building one of the experiments on the James Webb Space Telescope.
Free to join, register via Eventbrite on the link
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ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn
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lpm
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Re: Astronomy and Space

Post by lpm » Sat Dec 25, 2021 12:11 pm

I'm just about to launch this telescope thing. My finger's hovering over the button.

I'm streaming it live on YouTube.
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Martin Y
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Re: Astronomy and Space

Post by Martin Y » Sat Dec 25, 2021 2:39 pm

lpm wrote:
Sat Dec 25, 2021 12:11 pm
I'm just about to launch this telescope thing. My finger's hovering over the button.

I'm streaming it live on YouTube.
Nice job. Well done. Have a mince pie.

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Stranger Mouse
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Re: Astronomy and Space

Post by Stranger Mouse » Sat Dec 25, 2021 4:11 pm

I’ve decided I should be on the pardon list if that’s still in the works

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lpm
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Re: Astronomy and Space

Post by lpm » Sat Dec 25, 2021 5:22 pm

Martin Y wrote:
Sat Dec 25, 2021 2:39 pm
lpm wrote:
Sat Dec 25, 2021 12:11 pm
I'm just about to launch this telescope thing. My finger's hovering over the button.

I'm streaming it live on YouTube.
Nice job. Well done. Have a mince pie.
Ta. They should put English labels on and move the launch button well away from the self destruct button. It was almost an amusing anecdote.
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dyqik
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Re: Astronomy and Space

Post by dyqik » Sat Dec 25, 2021 5:59 pm


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bjn
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Re: Astronomy and Space

Post by bjn » Sat Dec 25, 2021 6:09 pm

Nice realtime tracker for the JWST, except for the lack of a fixed width font for the rapidly ticking readouts.

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Grumble
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Re: Astronomy and Space

Post by Grumble » Sat Dec 25, 2021 8:03 pm

bjn wrote:
Sat Dec 25, 2021 6:09 pm
Nice realtime tracker for the JWST, except for the lack of a fixed width font for the rapidly ticking readouts.
I like that, thanks
A bit churlish

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Re: Astronomy and Space

Post by Gfamily » Tue Dec 28, 2021 12:19 am

bjn wrote:
Sat Dec 25, 2021 6:09 pm
Nice realtime tracker for the JWST, except for the lack of a fixed width font for the rapidly ticking readouts.
Eh?
This is how they let you choose whether to use Miles/Fahrenheit or Km/Celsius
Webb.png
Webb.png (58.77 KiB) Viewed 1610 times
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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bjn
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Re: Astronomy and Space

Post by bjn » Tue Dec 28, 2021 12:21 am

At least they fixed the font problem.

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Martin Y
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Re: Astronomy and Space

Post by Martin Y » Tue Dec 28, 2021 12:59 am

Gfamily wrote:
Tue Dec 28, 2021 12:19 am
Eh?
This is how they let you choose whether to use Miles/Fahrenheit or Km/Celsius
Webb.png
The Americans call their customary weights and measures "English units" because that's what they derive from. We talk about "Imperial measure" but that isn't strictly the same and is a 19th century revision of English units.

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Re: Astronomy and Space

Post by dyqik » Tue Dec 28, 2021 1:17 am

Martin Y wrote:
Tue Dec 28, 2021 12:59 am
Gfamily wrote:
Tue Dec 28, 2021 12:19 am
Eh?
This is how they let you choose whether to use Miles/Fahrenheit or Km/Celsius
Webb.png
The Americans call their customary weights and measures "English units" because that's what they derive from. We talk about "Imperial measure" but that isn't strictly the same and is a 19th century revision of English units.
I call them Standard or US units.

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Grumble
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Re: Astronomy and Space

Post by Grumble » Tue Dec 28, 2021 8:07 am

dyqik wrote:
Tue Dec 28, 2021 1:17 am
Martin Y wrote:
Tue Dec 28, 2021 12:59 am
Gfamily wrote:
Tue Dec 28, 2021 12:19 am
Eh?
This is how they let you choose whether to use Miles/Fahrenheit or Km/Celsius
Webb.png
The Americans call their customary weights and measures "English units" because that's what they derive from. We talk about "Imperial measure" but that isn't strictly the same and is a 19th century revision of English units.
I call them Standard or US units.
Fair enough, but it must be fairly common for NASA to use English like that.
A bit churlish

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Re: Astronomy and Space

Post by dyqik » Tue Dec 28, 2021 4:13 pm

Grumble wrote:
Tue Dec 28, 2021 8:07 am
dyqik wrote:
Tue Dec 28, 2021 1:17 am
Martin Y wrote:
Tue Dec 28, 2021 12:59 am

The Americans call their customary weights and measures "English units" because that's what they derive from. We talk about "Imperial measure" but that isn't strictly the same and is a 19th century revision of English units.
I call them Standard or US units.
Fair enough, but it must be fairly common for NASA to use English like that.
It's common outside of the bits of engineering where there are lots of international people who know that the English don't use English units. So common in NASA PR, as it's known outside of NASA that way.

For specific things, other terms get used - NPT/NPS vs metric for plumbing, SAE vs metric for wrenches and bolts.

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Re: Astronomy and Space

Post by dyqik » Wed Jan 05, 2022 6:33 pm

JWST is now an assembled telescope, as the secondary mirror has been deployed and locked in place.

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Grumble
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Re: Astronomy and Space

Post by Grumble » Wed Jan 05, 2022 7:08 pm

dyqik wrote:
Wed Jan 05, 2022 6:33 pm
JWST is now an assembled telescope, as the secondary mirror has been deployed and locked in place.
If the main mirror fails to deploy properly will the half of the panels that are already in place give a reasonable image?
A bit churlish

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dyqik
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Re: Astronomy and Space

Post by dyqik » Wed Jan 05, 2022 8:10 pm

Grumble wrote:
Wed Jan 05, 2022 7:08 pm
dyqik wrote:
Wed Jan 05, 2022 6:33 pm
JWST is now an assembled telescope, as the secondary mirror has been deployed and locked in place.
If the main mirror fails to deploy properly will the half of the panels that are already in place give a reasonable image?
Yes, it won't be terrible. A bit oblate in the point spread function, and obviously a loss of sensitivity (fewer photons arriving) and a loss of resolution in one axis. There's also a loss of sensitivity and resolution that would happen if adjusting all of the panel positions after deployment failed.

Much of the instrumentation and science doesn't need great imaging, as it's spectroscopy work.

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Re: Astronomy and Space

Post by monkey » Wed Jan 05, 2022 8:48 pm

Grumble wrote:
Wed Jan 05, 2022 7:08 pm
dyqik wrote:
Wed Jan 05, 2022 6:33 pm
JWST is now an assembled telescope, as the secondary mirror has been deployed and locked in place.
If the main mirror fails to deploy properly will the half of the panels that are already in place give a reasonable image?
The width of point spread function, what gives you your resolution, is inversely proportional to the pupil diameter (the primary mirror in this case, also wavelength). For a nice round mirror/pupil you get a nice Airy disc, in the diffraction limited case*. If the mirrors on the side don't unfold, you'd pretty much have an elliptical pupil. This gives you your expected resolution in one direction, but lower in the other. If you lose the two outermost panels, your diameter is reduced by 2/5, so you'd expect the resolution to be 5/3 of expected in that direction**. This should still be slightly better than Hubble, but would be a significant degradation.

And if the mirror doesn't unfold, there may be issues with aligning the segments, which would mean aberrations won't be corrected, degrading the image.

So I imagine you'd still get a nice image, but it won't be as nice as it should be especially in one direction. There'd be less light too, so a loss of sensitivity.

It's also more complicated than that because it's made of hexagonal segments, so not perfectly round. If you want to try yourself, the PSF is given by the Fourier Transform of the pupil, and the convolution of that and your object gives you your image. Be careful with scaling.


*JWT isn't perfectly round, so the PSF won't be a perfect Airy Disc, but it is close enough to be considered diffraction limited, at a Strehl ratio > 0.8, IRC. In my work we normally consider >0.9 to be diffraction limited, but it's easier to do that on an optics table with much smaller elements than in space with Big Stuff. The Strehl ratio is a measure of how close your PSF is to a perfect Airy Disc.

**Resolution is one of those annoying ones where a bigger number is a reduction.

ETA: Or just what Dyqik said.
Last edited by monkey on Wed Jan 05, 2022 8:51 pm, edited 1 time in total.

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Grumble
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Re: Astronomy and Space

Post by Grumble » Wed Jan 05, 2022 8:51 pm

Thanks both of you.
A bit churlish

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Re: Astronomy and Space

Post by dyqik » Wed Jan 05, 2022 11:27 pm

monkey wrote:
Wed Jan 05, 2022 8:48 pm
*JWT isn't perfectly round, so the PSF won't be a perfect Airy Disc, but it is close enough to be considered diffraction limited, at a Strehl ratio > 0.8, IRC. In my work we normally consider >0.9 to be diffraction limited, but it's easier to do that on an optics table with much smaller elements than in space with Big Stuff. The Strehl ratio is a measure of how close your PSF is to a perfect Airy Disc.
The effects of Strehl ratio are a bit different for point sources on a dark background than for general imaging in terms of effect on the science.

We use 0.8 throughout astronomy, even down in the millimeter-wave bands where you really need to use Gaussian or physical optics, and where you have to worry a little bit about partial coherence.

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Re: Astronomy and Space

Post by monkey » Thu Jan 06, 2022 4:03 pm

dyqik wrote:
Wed Jan 05, 2022 11:27 pm
monkey wrote:
Wed Jan 05, 2022 8:48 pm
*JWT isn't perfectly round, so the PSF won't be a perfect Airy Disc, but it is close enough to be considered diffraction limited, at a Strehl ratio > 0.8, IRC. In my work we normally consider >0.9 to be diffraction limited, but it's easier to do that on an optics table with much smaller elements than in space with Big Stuff. The Strehl ratio is a measure of how close your PSF is to a perfect Airy Disc.
The effects of Strehl ratio are a bit different for point sources on a dark background than for general imaging in terms of effect on the science.

We use 0.8 throughout astronomy, even down in the millimeter-wave bands where you really need to use Gaussian or physical optics, and where you have to worry a little bit about partial coherence.
I've always found Strehl cut-offs to be arbitrary, anyway. It's not important that a system is good enough to get over a threshold, it's important that it's good enough to do the job you want it to.

I prefer to see what the wavefront is doing at the pupil, but I work with adaptive optics, so we measure that and that makes it easy to see. In my field, Strehl is only quoted when someone wants to make a claim that their system is diffraction limited (as I kind of did above!), but everyone seems much more proud about how flat the wavefront is in their system design.

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Re: Astronomy and Space

Post by dyqik » Fri Jan 07, 2022 11:00 pm

Grumble wrote:
Wed Jan 05, 2022 7:08 pm
dyqik wrote:
Wed Jan 05, 2022 6:33 pm
JWST is now an assembled telescope, as the secondary mirror has been deployed and locked in place.
If the main mirror fails to deploy properly will the half of the panels that are already in place give a reasonable image?
Half of it has now deployed and locked in place, so this is already less of a concern.

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Grumble
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Re: Astronomy and Space

Post by Grumble » Sat Jan 08, 2022 8:12 am

It’s interesting to see the temperature difference between positions C and D coming down. At -199 and -172C right now. I assume when operating you want those to be near enough identical
A bit churlish

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Re: Astronomy and Space

Post by TimW » Tue Jan 11, 2022 12:11 pm

the BBC wrote:Webb was funded for an initial five years of operations with an expectation it could work for about 10. The life-limiting factor has always been considered to be the amount of fuel onboard the telescope to maintain positioning in space. But such was the accuracy of the launch rocket in putting Webb in just the right part of the sky, astronomers can now look forward to a much longer lived observatory.

"We have quite a bit of fuel margin right now relative to 10 years. Roughly speaking, it's around 20 years of propellant," said Mike Menzel, Nasa's Webb mission systems engineer.

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