Astronomy and Space
Re: Astronomy and Space
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.
That is a lot of launches.
Re: Astronomy and Space
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
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ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn
Meta? I'd say so!
ETA 5/8/20: I've been advised that the result was correct, it was the initial interpretation that needed to be withdrawn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!
Re: Astronomy and Space
I'm just about to launch this telescope thing. My finger's hovering over the button.
I'm streaming it live on YouTube.
I'm streaming it live on YouTube.
Awarded gold star 4 November 2021
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Re: Astronomy and Space
Free book which may be of interest https://www.amazon.co.uk/Astronomy-Andr ... B075FG4KTK
I’ve decided I should be on the pardon list if that’s still in the works
Re: Astronomy and Space
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.
Awarded gold star 4 November 2021
Re: Astronomy and Space
Nice realtime tracker for the JWST, except for the lack of a fixed width font for the rapidly ticking readouts.
Re: Astronomy and Space
I like that, thanksbjn wrote: ↑Sat Dec 25, 2021 6:09 pmNice realtime tracker for the JWST, except for the lack of a fixed width font for the rapidly ticking readouts.
where once I used to scintillate
now I sin till ten past three
now I sin till ten past three
Re: Astronomy and Space
Eh?bjn wrote: ↑Sat Dec 25, 2021 6:09 pmNice realtime tracker for the JWST, except for the lack of a fixed width font for the rapidly ticking readouts.
This is how they let you choose whether to use Miles/Fahrenheit or Km/Celsius
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!
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!
Re: Astronomy and Space
At least they fixed the font problem.
Re: Astronomy and Space
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.
Re: Astronomy and Space
I call them Standard or US units.
Re: Astronomy and Space
Fair enough, but it must be fairly common for NASA to use English like that.dyqik wrote: ↑Tue Dec 28, 2021 1:17 amI call them Standard or US units.
where once I used to scintillate
now I sin till ten past three
now I sin till ten past three
Re: Astronomy and Space
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.
Re: Astronomy and Space
JWST is now an assembled telescope, as the secondary mirror has been deployed and locked in place.
Re: Astronomy and Space
If the main mirror fails to deploy properly will the half of the panels that are already in place give a reasonable image?
where once I used to scintillate
now I sin till ten past three
now I sin till ten past three
Re: Astronomy and Space
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.
Re: Astronomy and Space
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.
Re: Astronomy and Space
Thanks both of you.
where once I used to scintillate
now I sin till ten past three
now I sin till ten past three
Re: Astronomy and Space
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.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.
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.
Re: Astronomy and Space
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.dyqik wrote: ↑Wed Jan 05, 2022 11:27 pmThe 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.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.
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 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.
Re: Astronomy and Space
Re: Astronomy and Space
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
where once I used to scintillate
now I sin till ten past three
now I sin till ten past three
Re: Astronomy and Space
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.