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Why Hubble was cancelled, and what to do now
Since Ken Arromdee fairly asked what really killed Hubble, I found an
honest-looking answer in the notes from a meeting between Sean O'Keefe and Hubble's managers at the Space Telescope Science Institute: http://www.stsci.edu/institute/sm4meeting.html My summary: O'Keefe cancelled the Hubble service mission to satisfy two masters, President Bush and the CAIB, neither of which were anti-Hubble. Bush set a limit to the shuttle program, while CAIB demanded new safety requirements. If you fit ISS service flights between Bush and CAIB, there is no room left for the Hubble flight. Hubble would have required special safety rituals and NASA can no longer afford them. Even though everyone realizes that it sucks, the Hubble service flight was sacrificed to politics. The space station is a political caricature of science, the shuttle EVA repair rules are a political caricature of safety, and the Bush plan is a political caricature of a space agency plan. Together they killed Hubble. So what to do now? If you like space telescopes, do not cry for Hubble. Instead of asking Washington for a reprieve, you should ask for guaranteed funding for the James Webb Space Telescope. After all, it used to be called the Next Generation Space Telescope, meaning the next generation after Hubble. Each of the 18 main mirrors of JWST will be about the same size as the entire main mirror of Hubble. Here is a rendering: http://www.stsci.edu/jwst/overview/i...lls_SK_sm1.jpg This is the reflection telescope that astronomers really want. Granted, the "save the Hubble" camp argues that JWST is an infrared telescope, not optical. But why should it be? The optical band is one of the few narrow bands for which the atmosphere is transparent. (It's opaque to near infrared, incandescent in far infrared, and badly translucent beyond near ultraviolet.) Astronomers are planning a ground-based mirror, the Thirty-Meter Telescope, with 25 times the area of JWST. It should be better than Hubble in the optical range. -- /\ Greg Kuperberg (UC Davis) / \ \ / Visit the Math ArXiv Front at http://front.math.ucdavis.edu/ \/ * All the math that's fit to e-print * |
#2
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Why Hubble was cancelled, and what to do now
In article ,
Greg Kuperberg wrote: Granted, the "save the Hubble" camp argues that JWST is an infrared telescope, not optical. But why should it be? The optical band is one of the few narrow bands for which the atmosphere is transparent. Kind of, sort of, loosely speaking. There are still major advantages to being above it. And I notice you don't mention ultraviolet. Astronomers are planning a ground-based mirror, the Thirty-Meter Telescope, with 25 times the area of JWST. It should be better than Hubble in the optical range. Better for some things, not for everything. And it won't do UV. -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
#3
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Why Hubble was cancelled, and what to do now
In article ,
Henry Spencer wrote: In article , Greg Kuperberg wrote: Granted, the "save the Hubble" camp argues that JWST is an infrared telescope, not optical. But why should it be? The optical band is one of the few narrow bands for which the atmosphere is transparent. Kind of, sort of, loosely speaking. There are still major advantages to being above it. Sure, loosely speaking. I guess I was speaking around the real point: The narrow optical band is very relevant for human physiology, and therefore for popular astronomy. It isn't very special for research-level astronomy. Moreover, it's one of the more favorable bands for ground-based telescopes, because people and telescopes are adapted to the same atmosphere. In this sense, the popular and practical attributes of a space telescope are contradictory. And [TMT on the ground] won't do UV. Yes, Hubble is a great UV telescope. I suppose that that capability might not be replaced for a long time. Still, the infrared JWST is what the STScI people chose as the "next generation". I'm sure that they had good research reasons for that. (Actually "infrablue" might be a better term for JWST. Supposedly its detectors will extend through green light.) -- /\ Greg Kuperberg (UC Davis) / \ \ / Visit the Math ArXiv Front at http://front.math.ucdavis.edu/ \/ * All the math that's fit to e-print * |
#4
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Why Hubble was cancelled, and what to do now
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#5
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Why Hubble was cancelled, and what to do now
In article ,
Allen Thomson wrote: (Greg Kuperberg) wrote (Actually "infrablue" might be a better term for JWST. Supposedly its detectors will extend through green light.) 600 nm as of last word. 600 nm is labelled as yellow he http://www.gamonline.com/catalog/col...y/spectrum.gif According to this spec page, the different JWST cameras will be sensitive from 28,000 nm to 600 nm: http://www.stsci.edu/jwst/overview/design/ Actually this is an unclear point for me. The main JWST camera, NIRCam, appears to be almost convenient enough for the gee-wow color photographs, but not quite. It will be sensitive to 600 nm - but why not extend that to 360 nm for the sake of public relations? I suppose that they could make false-color pictures by permuting the spectrum, and maybe they would look even better than true-color. But NIRCam also only has two channels, rather than 3. Maybe they are used to faking three independent colors with two? Or maybe for public relations they plan to use the other camera, MIRI, which has 12 filters. -- /\ Greg Kuperberg (UC Davis) / \ \ / Visit the Math ArXiv Front at http://front.math.ucdavis.edu/ \/ * All the math that's fit to e-print * |
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Why Hubble was cancelled, and what to do now
Greg Kuperberg wrote:
Actually this is an unclear point for me. The main JWST camera, NIRCam, appears to be almost convenient enough for the gee-wow color photographs, but not quite. It will be sensitive to 600 nm - but why not extend that to 360 nm for the sake of public relations? Perhaps the optics may not be good enough for high resolution at 360 nm? Paul |
#8
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Why Hubble was cancelled, and what to do now
In article ,
Stephen Souter wrote: In article , (Greg Kuperberg) wrote: Astronomers are planning a ground-based mirror, the Thirty-Meter Telescope, with 25 times the area of JWST. It should be better than Hubble in the optical range. But wouldn't that increase the bill for the JWST... Rather, the TMT telescope should be better than Hubble in the optical range - albeit only calm days. JWST will be much better than Hubble in the range that it sees, namely far infrared to yellow light according to the current spec. I think that part of the point of JWST is that its shifted frequency range, which is actually broader than that of Hubble on a logarithmic scale, is more interest to astronomers than Hubble's range. In any case, I notice that the JWST only has a 5-10 year lifespan (versus 20+ years for Hubble). Since the JWST is presumably not being designed to be serviced in situ,... First of all JWST will be farther away than the moon. Thus, so-called "service missions" are out the window. Again, the wisdom of the astronomy community is that LEO sucks for most space telescopes, certainly for infrared ones. Second, the "service mission" plan for Hubble turned out to be a great Faustian bargain for astronomers. It was pursued largely to the exclusion of launching other space telescopes. Hubble is like an old computer data center with a 20-year service contract. Sure, they might get upgrade the memory and even the CPU now and then. Of course what people really want is completely new computers every few years. And likewise astronomers want completely new telescopes. It may be counterintuitive for a lot of people here to think of a billion-dollar spacecraft as disposable. Well, even on the ground each giant new particle accelerator makes the old one almost obselete. And CPU chip factories face a similar fate. -- /\ Greg Kuperberg (UC Davis) / \ \ / Visit the Math ArXiv Front at http://front.math.ucdavis.edu/ \/ * All the math that's fit to e-print * |
#9
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Why Hubble was cancelled, and what to do now
In article ,
Greg Kuperberg wrote: First of all JWST will be farther away than the moon. Thus, so-called "service missions" are out the window... If you've got a system that could do a servicing visit in lunar orbit, then with a bit less propulsion and a bit more life support, it could do one at L2. It's significantly hard but not ridiculous, if the cost is low enough. Again, the wisdom of the astronomy community is that LEO sucks for most space telescopes, certainly for infrared ones. If nothing else, most telescopes are better off without Earth filling half the sky and getting in the way constantly. Whether that's enough to justify going farther out depends on details, and in particular on whether there are other reasons as well. Certainly there are for infrared (colder environment) and for high-energy astronomy (less particle noise from the Van Allen belts). ...Hubble is like an old computer data center with a 20-year service contract. Sure, they might get upgrade the memory and even the CPU now and then. Of course what people really want is completely new computers every few years. And likewise astronomers want completely new telescopes. Keck must be on its last legs by now, to say nothing of ridiculous antiques like almost everything on Mount Palomar. Only in space is it accepted that you throw out a hugely expensive observatory after a few years of use. Anywhere else, it may not be right at the leading edge any more, but it remains in service. It may be counterintuitive for a lot of people here to think of a billion-dollar spacecraft as disposable. Well, even on the ground each giant new particle accelerator makes the old one almost obselete. On the contrary. The new ones are built as extensions of the old ones whenever possible -- e.g., the Tevatron is fed by the original Fermilab ring (and yes, there have been proposals to do similar things with Hubble!) -- and even when that's not done, often the older machines are rebuilt for secondary roles. -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
#10
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Why Hubble was cancelled, and what to do now
"Paul F. Dietz" wrote in message ...
Greg Kuperberg wrote: Actually this is an unclear point for me. The main JWST camera, NIRCam, appears to be almost convenient enough for the gee-wow color photographs, but not quite. It will be sensitive to 600 nm - but why not extend that to 360 nm for the sake of public relations? Perhaps the optics may not be good enough for high resolution at 360 nm? JWST's mirror will be actively controlled. In order to have a decent optical mirror it must be within a small tolerance of the ideal optical shape. The deviation of a mirror from the ideal is often quoted as a fraction of some wavelength (usually representing the average wavelength used in observations with the instrument). For a recreational or backyard telescope deviations of around 1/4 Lambda (wavelength) are usually acceptable. Lambda/8 would be decent, Lambda/12 or Lambda/16 would be research grade. For an actively controlled mirror surface there are huge costs to pay for choosing a smaller Lambda (going for visual or ultraviolet instead of IR). First, each actuator has to be all that much more fine, for near-IR to visible light that can make a difference of around a factor of 2 or so, which is no small thing for actuators which are already insanely precise and expensive. Second, you've got to increase the actuator density to shore up the "sagging" in between actuators (though it might be "sagging" too high or too low). The number of actuators needed scales roughly with the inverse square of Lambda. Decrease Lambda by 2 (1,000 nm to 500 nm, for example) and you multiply the number of actuators needed by about 4, and they have to be 2x as fine. That drives up cost and complexity, and also drives up the areal mass of the mirror. Which is a rather big issue, sadly, with the launch capabilities we have today. Especially since JWST is developing this technology as it goes, it makes a hell of a lot of sense to shoot for a lower goal first and, maybe, let incremental improvements and evolution take the technology to where it might prove useful in visible and UV wavelengths later on. |
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