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Space Science

Ground-based Telescope as Sharp as Hubble 36

Posted by michael from the corrective-lenses-required dept.
Midnight Thunder writes: "The BBC has an article describing how the Paranal Observatory has been able to take images that are just as sharp as the Hubble Space Telescope. For a ground based telescope the images are of amazing quality."
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Ground-based Telescope as Sharp as Hubble More

Ground-based Telescope as Sharp as Hubble

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  • Corrective lenses? (Score:3, Insightful)

    by Liquor ( 189040 ) on Tuesday December 04, 2001 @03:10PM (#2655016) Homepage
    It's interesting to note that the Paranal telescope modifies the mirror to correct for an imperfect lens (the atmosphere), while the Hubble has a corrective lens (installed in orbit) in it's system to correct for a manufacturing error in the mirror.

    It makes me wonder if the Hubble would have been significantly better than Paranal if the mirror had been made correctly in the first place.

    • Re:Corrective lenses? (Score:3, Insightful)

      by LMCBoy ( 185365 )
      It's not a good analogy, because the correction used by Paranal is an adaptive optics system that continually updates the shape of the mirror 500 times per second to correct for the turbulent atmosphere, which causes images to bounce around in the focal plane (a/k/a the "twinkling" of stars). OTOH, the HST correction is static aberration correction, and therefore much simpler, technologically. HST doesn't need adaptive optics, because there is no atmosphere between it and its targets (that was the whole point of putting it in orbit).

      Furthermore, even if the HST mirror had been manufactured perfectly, it would be no better than the post-corrected HST. In other words, the HST fix made it "as good as new".

      Theoretically, once you have perfect optics, and have corrected for the atmosphere perfectly (if you're on the ground), the sharpest image you can achieve is limited by quantum mechanics; it's known as the diffraction limit. The size of a diffraction-limited point source is inversely proportional to the diameter of the aperture (i.e., the primary mirror).

      Since Paranal is a much larger telescope than HST, (8.2 meteres compared to 0.9), it's ideal, diffraction-limited image is much sharper than HST's. The fact that they can "only" get as good as HST shows you how hard adaptive optics is.
      • True, I was comparing apples to oranges here - but there are some similarities.

        With adaptive optics, unless the atmosphere is completely predictable, there is going to be a lag between observing the distortion and correcting the distortion that limits the perfection of the result.

        Similarly, with the HST mirror - the corrective lens corrects for the systematic error in the manufacture of the mirror, but does nothing for the residual (local) errors in the original mirror, and indeed, any residual errors in it's own manufacture will be added to them.

        The diffraction limit can only be approached when the accuracy of the entire system is perfectly consistent to a fraction of a wavelength - that Paranal is even that close to the resolution of the smaller HST is a tribute to how well the adaptive optics are implemented.

        On the other hand, the Hubble - assuming a correctly made mirror - does not have to deal with either atmospheric distortion or gravity/motion compensation, and theoretically would only be limited by the diffraction limit.

        I suspect that, once the Paranal telescopes are indeed linked, or the adaptive optics are better tuned, that Paranal may eventually be better than the Hubble at times, especially when it comes to resolving faint objects. (It should gather about 80 times the light that the Hubble can.)

        But the HST certainly sets a high standard to be reached.
      • Furthermore, even if the HST mirror had been manufactured perfectly, it would be no better than the post-corrected HST. In other words, the HST fix made it "as good as new".


        The extra lense introduces a data loss. It will be removed in 2003 [ball.com].

        .
        • "The extra lense introduces a data loss. It will be removed in 2003"

          Yeah, okay, no optic is perfect, but the loss caused by COSTAR is not significant.

          Just FYI, while COSTAR won't be physically removed until 2003, it was permanently retracted out of the optical path a few weeks ago, in anticipation of the removal of the faint object spectrograph early next year (the last instrument that doesn't internally correct for Hubble's aberration).

          cheers,
          Jason
    • I did a lame research paper and classroom presentation on the Hubble Space Telescope way back in the day (read: I'm pretty sure of my info, but I'm not sure where to look for verification currently). Anyway, the result of my little grade-school investigations were that the in-orbit modifications they made improved HST significantly beyond its initial design. Actually, a little bit of looking turned up this: HST Servicing Missions [stsci.edu]. It's rather dumbed-down, but I think it'll help you draw comparisons.

      In any case, there are a few things you should keep in mind. First, HST is quite old now, and past its initial proposed service lifetime, IIRC, so technology has come a long way since it went up. Second, things like this are often technologically lagging even before they go up, since it can take literally decades to plan an instrument of that size. Size is the other critical thing to keep in mind. It's (relatively) easy to build huge arrays like VLT [eso.org] on the ground, but try getting that into orbit :). Even if you break it up into many missions and assemble in-orbit (some early plans for NGST [stsci.edu] considered that, IIRC), it would still be a monumental task (though not impossible).

      So I'd say in summary that chances are HST never could have been better than Paranal, even though it's been an awesome instrument throughout its lifetime.

  • Huge News (Score:2, Interesting)

    by Fraser Cain ( 203191 )
    This is enormous news when you consider the bottleneck of time the Hubble Space Telescope has become. If it works well, you can expect this technology to be applied to many ground-based observatories.

    Then multiply it with the technology that merges the images from several mirrors to act like one giant mirror.

    Finally, when you match this technology with the new technique devised to detect the atmospheres of distant planets, it really offers a lot to planet hunters.

    I think this will revolutionize planet hunting, and bring the detection of Earth-sized planets with oxygen atmospheres within the near future.

    Fraser Cain

    • Not so sure... (Score:2, Interesting)

      by QuantumFTL ( 197300 )
      IANAA, but I have a few comments neverthe less :)

      First of all, visible light just isn't the best spectrum to do astronomy in for a lot of things, especially not the detection of extrasolar planets. Infrared radiation, unhindered by most space dust, and lower in energy, is clearly superior [caltech.edu] for studying things that are not giant balls of gas. The Next Generation Space Telescope [nasa.gov] and the Terrestrial Planet Finder [nasa.gov] both use infrared radiation to study objects of great interest that are difficult to study with something like the HST.

      Interferometry, the technology you refer to that allows telescopes to combine their phase information to generate an image with angular resolution of that of a single larger telescope (through something known as apature synthesis) is only one of the many uses of intereferometry. Perhaps much more exciting than that is the ability of the Terrestrial Planet Finder to use nulling interferometry to selectively block out the radiation from a star, without blocking out the much fainter (millions of times less) glow of a circling planet.

      Unfortunately the earth's atmosphere is mostly opaque to infra-red light, and room temperature objects (like most of the surface of the earth, and the telescopes on it) generate so much infra-red radiation that it makes it nearly impossible to do any far-infrared studies from the ground. The Darwin Project [esa.int] web site has a good explaination about the reasons terrestrial planet hunting should be done in space.

      Ground based observatories will always have a place, however eventually it will be a matter of cost and convenience rather than any technical superiority.

      Not saying this isn't cool, but it's mostly postponing the inevitable day when very little new astronomy can be done inside the confines of an atmosphere....

    • his is enormous news when you consider the bottleneck of time the Hubble Space Telescope has become. If it works well, you can expect this technology to be applied to many ground-based observatories.

      They've tried many times to apply this and other technologies to ground based telescopes. The problem is that they've been very expensive and very high maintenance.
  • Image processing and cheap CCD detectors have REEEEALLLLYYY improved astro-imaging from the ground. In fact, it's restarted a dead line of astronomy: studing images instead of spectra and other forms of data.

    Amateurs are doing amazing stuff. Here's an image of Saturn [core.com] taken with an amateur 13-inch scope and a camcorder. It's compared side-to-side with a similar HST image. You will be surprised.

    Dozens of amateurs joined in a program to supply images to the 2001 International Marswatch [rowan.edu] program during this past Martian observing season. The pros use these images to decide when to spend their valuable HST time to look at Mars. Some of the images (and visual drawings) are incredible.

    • What's really cool is that adaptive optics technology is already available for the amateur. Santa Barbara Instrument Group [sbig.com] makes the AO-7 adaptive optics system that works with their ST-7 and ST-8 CCD imaging cameras. It retails for about $1200.

    • Nice shot for a home telescope but it's a small fraction of the resolution on the instrument you chose to compare with, and it can't match the capability of the other instruments on the Hubble. It's downright misleading to draw this comparrison.

      Saturn is the easiest object to image, you chose it for a reason. How about some of the feinter objects. Come on show us just how incapable your 13 inch telescope really is with a deep field shot.
      • Sorry, but it's not my telescope or image and I don't do any serious CCD imaging with the scopes that I do have :-)

        Nonetheless, amateur built CCD's can do pretty amazing things - they can go deeper with a with a couple of minutes of exposure time then the 200-inch Hale Telescope could using film in 1970. Here's a good example, look at these images of extremely faint Globular clusters [angelfire.com] taken with a homemade telescope and CCD camera.

        Some of these objects weren't recorded on the Palomar Sky Survey!

        Understand, ground based telescopes aren't a replacement for space based telescopes, if not just because of the bandwidth issues ground based scopes face. Nonetheless, ground based professionals and amateurs are recording data that was believed to have been the sole terrority of space based telescopes when HST was designed, built and launched.

        NASA's smart by building NGST [nasa.gov] to primarly work in the IR ranges.

        Oh, BTW, Jupiter and the moon are easier to image then Saturn because they are brighter and larger... :-)

  • .. on a show about super black holes..

    Aparently hubble which exists in space was so powerful at the time that it was put up there and it has the advantage that it does not have to go throuht the atmosperic distortion. However this new one is built on a mountain and is more powerful (newer technology) and is able to 'see' black holes at the center of galaxies. It seems that all galaxies have black holes, but not all black holes are feeding too. Of course the one at the center of our galaxy is feeding again... but don't worry, it wont eat us, at least not until after the andromedia galaxy collides with the milkyway in a few million years...

    • As I recall Andromeda is supposed to collide with us in 200 million years, give or take a couple million. Because the outer regions of galaxies are rather sparse I suspect other than the light show in the sky we (or whatever might be looking up at the sky 200 million years from now) will probably be relatively unaffected.

      Re: super-telescopes, I cannot wait for the NASA Origins [nasa.gov] programs' second and third generation telescopes, the planet finders [nasa.gov]. The goal by 2020 is to put high-power optical interferometers in space, so we can not only infer the presence of planets, but image them.
    • Yep, you are right (apart form the time-frame as others have pointed out), unless the Sun takes a near direct hit, the andromeda and our galaxy would pass each other without much fuss (at least in our part of it).

      Granted, the shapes of the two galaxies would change dramatically, and perhaps the sun would be thrown out of the milkyway, but if that happened, the rest of the solar system vould follow, and the inner solar system would be largely unaffected.

      The Oort cloud of snowballs (e.g. comets) would be affected though. So the inner solar system would experince some fancy fireworks in the sky when a lot of comets starts to appear. These comets may collide with earth (big 'BOOM' as on Jupiter a few years ago) but otherwise we (or who may be here at that time) would not notice.


      Yours Yazeran


      Plan: To go to Mars one day with a hammer.

      • actualy according to the show, it depends on where the earth / sun is at the time of collision. If we are on one side we would feed the blackholes at the center and earth would die. If we are at the other side then we would get throw out as you describe.

        Then again there is the change that earth would be somewhere in between and survive unharmed. I'd be suprised if the human race was still around then and evolution hadn't had another species tak our place.

  • This is not new (Score:3, Informative)

    by lanclos ( 150352 ) on Tuesday December 04, 2001 @06:48PM (#2656582) Homepage
    Keck [hawaii.edu]'s been capable of "beating" Hubble for a good long while now. Adaptive Optics [ucolick.org] is wild and crazy stuff.

    Please don't believe that we'll be able to do away with space-based observing because of this innovation. Our atmosphere absorbs an awful lot of interesting wavelengths.
    • Please don't believe that we'll be able to do away with space-based observing because of this innovation. Our atmosphere absorbs an awful lot of interesting wavelengths.
      Short and long IR for example. But check out the Sofia [nasa.gov] project, mounting a telescope on a plane can get you above 98% of the IR absorbing water vapor for significantly less cost than a space based telescope. -Rob

    • Re:This is not new (Score:1, Informative)

      by Anonymous Coward
      Indeed, adaptive optics has been around for a few years at most observatories. However, it is still being developed.

      I have used the adaptive optics systems at both Keck and Lick observatories. They can work very well, but are generally limited to observations near a bright (V~13 or so within ~40 arcsec) guide star. This limit means that only ~1% of the sky is accessible. A laser tuned to a sodium line can be used to create an artificial guide star. In this case, you still need a star for "tip-tilt" correction, but it can be fainter (roughly V~16).
      Laser guide star AO is still in development at Lick (though it does work -- I have a small amount of data obtained with it), and not yet available at Keck.

      There are also issues with the point spread function and dynamic range, especially if you are observing off-axis of the guide star.
      So don't give up on old HST just yet...
      -- Just Another Random Astrophysicist
    • One other advantage of space-based telescopes (not realized by Hubble since it's so close to Earth) is the much longer observing times you can have, viewing a region of the sky for possible days at a time. On Earth's surface you're limited to night-time observations, but in space pointing away from the Sun it's always night.
  • What we're observing is technical "stair-stepping."

    At the time Hubble was conceived, ground telescope technology had run into a brick wall, specifically, atmospheric distortion. The solution? Well, fly above the atmosphere, and that's how Hubble was born.

    25+ years since the era in which Hubble was designed, we now see that computers, optics, and control mechanisms have advanced to near-Asimov proportions - this has produced the ability to make a mirror that can compensate for the earth's atmosphere and optically eliminate its effects. So now, the Paranal facility (Keck, and others) can claim optical superiority over Hubble.

    The next step, of course, is to apply similar anti-distortion and image linking techniques to a spaceborne observatory. Call this "Hubble 2" (or "Son of Hubble," "Hubble TNG," "Hubble AOTC," whatever fits).

    The issue of image perfection and space orbservatories is a matter of economics - it's always tougher to build things to go into space rather than similar ground-based systems, and so Hubble 2 might be a long time coming. When it arrives, though, Paranal will just seem like chaffe.
    • To what would this hypothetical adaptive space telescope adapt? The adaptive ground-based telescopes adapt to the atmosphere; but there's no atmosphere in space.

      I just don't see how adaptive technology could improve a space telescope.

      -Billy
      • It couldn't. The solution in space is to make bigger telescopes. This is a big ground-based telescope and it's taking the finest electronics we've got to make it measure up to a much smaller one in space. Remember, gravity isn't an issue up there so, say, a 100 meter lens would be easier to construct and deploy.

        Of course, to do that we'd need a developed industrial capacity beyond the clouds, which means we'd need decent launching capabilities. Which means NASA won't have any part of it.

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