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James Webb Space Telescope, information, updates and discussion
Steve G
post Nov 3 2016, 03:44 PM
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With the launch 2 years away, I've tried to find out what the Solar System observation resolution will be on Webb, and I need it broken down to very basic layman's terms.

ON LINE: How good is the angular resolution?

CONVOLUTED ANSWER: The specification is that the telescope is diffraction limited at 2 μm, which means a Strehl ratio of 0.8 and a wavefront error of 150 nm rms. With a 6.5 m telescope, 1.22 λ/D = 0.077 arcsec at 2 μm. The smallest pixels (NIRCam 0.6-2.5 μm) are just 0.034 arcsec. But a lot of the wavefront error is due to imperfect alignment of the parts, and it's possible to do better for a small part of the field of view

For a space enthusiast, not an astronomer, this is like converting kilo-Newtons in to pounds of thrust (and who decided to switch from a very simple and easy to understand Lbs of thrust to a kilo-Newton as if anyone is expected to know what a kilo-Newton is) . So what kind of resolution can we expect for Mars, or objects in the asteroid belt such as Pallas, or Io, or Triton, for example? How many pixels across or resolution per pixel pair? Something that is in plain English would be awesome.

Thanks.




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elakdawalla
post Nov 3 2016, 03:56 PM
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One issue I hadn't appreciated until recently is how the design of JWST isn't exactly optimized for solar system observations. It has to keep the sun shield between it and the sun, but the telescope points at right angles to the sun shield. So it can't point at things when they are at opposition, only at geometries roughly tangent to JWST's orbit. Doesn't matter as much for resolution on distant targets but makes a big difference for Mars and asteroids, and limits when things can be observed.


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JRehling
post Nov 3 2016, 05:40 PM
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QUOTE (Steve G @ Nov 3 2016, 08:44 AM) *
[...]
The smallest pixels (NIRCam 0.6-2.5 μm) are just 0.034 arcsec.
[...]
For a space enthusiast, not an astronomer, this is like converting kilo-Newtons in to pounds of thrust (and who decided to switch from a very simple and easy to understand Lbs of thrust to a kilo-Newton as if anyone is expected to know what a kilo-Newton is) . So what kind of resolution can we expect for Mars, or objects in the asteroid belt such as Pallas, or Io, or Triton, for example? How many pixels across or resolution per pixel pair?


I honestly never thought much about angular sizes until I started doing astrophotography, and then it becomes absolutely essential and I can't imagine anything more fundamental. If you give someone the resolution for Mars, then they're stuck with just that, and have to convert it to any other object they care about… and, JWST is not primarily for solar system objects, and many cosmic objects are of unknown distance, so for those cases, we can't compute resolution in absolute distances and angular diameter is all we have.

That said, Mars covered about 18.5 arcsec during this opposition, so for the instrument quoted above, Mars would be about 540 pixels across, so a JWST pixel on Mars would be about 12.5 km wide. Ganymede would be about 50 pixels wide, with pixels roughly 100 km wide.

More relevant, that translates into pixels about 6.3 million km = 0.04 AU wide at Proxima Centauri and Alpha Centauri. That is coincidentally almost exactly the orbital radius of Proxima b, which tells you that direct observation of planets in the "habitable zone" of red dwarfs is going to be very difficult with telescopes, even in that best-case scenario. Planets in the habitable zone of sunlike stars 100 light years away would have the same angular separation from their primary as Proxima b, but there are a lot of stars closer than that, so JWST is going to give us an all-new capability for direct visual observation of exoplanets… but what can be learned from such observations is still an open question.

The ground-based E-ELT, however, will have much better (more than ten times better) resolution than JWST, so it will be the ace instrument for fine resolution when it comes online.

JWST is not going to be the ultimate telescope for high resolution and, while I'm sure it will make some great solar system observations, it's not designed for any such purposes. Its distinguishing characteristics will be its abilities to observe through cosmic dust, heavily red-shifted objects, and exoplanets. The first two of those address major blindspots we have – about 20% of the deep sky is hidden by the Milky Way, and lots of the "back" sides of deep sky objects are hidden by their own front sides. And cosmic structures whose light left them in the first ~500 million years after the Big Bang are going to be seen much better by JWST than anything else. For solar system objects, JWST will not be a great upgrade in sheer resolution from existing telescopes, and its resolution will within a decade be utterly blown away by new, massive telescopes on Earth.
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SteveM
post Nov 12 2016, 02:19 PM
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QUOTE (elakdawalla @ Nov 3 2016, 10:56 AM) *
One issue I hadn't appreciated until recently is how the design of JWST isn't exactly optimized for solar system observations. It has to keep the sun shield between it and the sun, but the telescope points at right angles to the sun shield. So it can't point at things when they are at opposition, only at geometries roughly tangent to JWST's orbit. Doesn't matter as much for resolution on distant targets but makes a big difference for Mars and asteroids, and limits when things can be observed.
Thanks for pointing that out. A positive aspect of that geometry is that JWST can only observe stars six months apart when the line of sight is tangent to the Earth's orbit. That's the ideal geometry for stellar parallax measurements. With appropriate analyses of the stars' point spread functions, JWST might get up to the level of Gaia's parallax measurements.
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Holder of the Tw...
post Dec 22 2016, 02:31 PM
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Uh-oh!

Engineers examine unexpected readings from JWST shake test
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B Bernatchez
post Dec 22 2016, 08:20 PM
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I don't imagine that they crank up the shaker table to 11 until later in the test sequence. I would hope that this was discovered early on at a low vibration setting. Hopefully nothing more than an incorrect test setup.
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JRehling
post Dec 28 2016, 11:37 PM
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It sounds like the anomalous vibration at least did no harm:

http://phys.org/news/2016-12-nasa-webb-tel...on-anomaly.html

Switching subtopics mid-post, I've read some more encouraging things about JWST's expected ability to observe exoplanets, including Proxima B. If all is well on the engineering side, those results may be only ~3 years away.
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Holder of the Tw...
post Jan 26 2017, 02:15 PM
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Good news. The problem has been resolved ...

Vibration Testing to Resume
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Holder of the Tw...
post Oct 6 2017, 06:46 PM
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One more (hopefully final) launch delay. No one thing in particular, just a lot of little things adding up. They claim there will be no cost penalties as a result.

Early 2019 launch
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