James Webb Space Telescope, information, updates and discussion |
James Webb Space Telescope, information, updates and discussion |
Aug 23 2005, 02:01 PM
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Member Group: Members Posts: 134 Joined: 13-March 05 Member No.: 191 |
The manufacture of the JWST mirror blanks has now been completed.
Despite this milestone, the fate of JWST is still somewhat precarious, because although the scientific bang from the telescope is expected to be huge, the bucks required have increased to a staggering $4.5 billion. A Space.com article on the squeeze in NASA's space-based astronomy plans gives some background. The JWST home page can be found here. The Space Telescope Science Institute, which runs Hubble, also has a site here. As does ESA. |
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Nov 3 2016, 03:44 PM
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Member Group: Members Posts: 292 Joined: 29-December 05 From: Ottawa, ON Member No.: 624 |
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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Nov 3 2016, 05:40 PM
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
[...] 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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