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Space Science Technology

A Ground-Based Scope That Flexes For Better Focus 29

Posted by timothy from the much-cheaper dept.
Steve0987 writes "EE Times Online has an interesting article on a deformable telescope mirror that the University of Arizona has built. It uses 336 magnetic coils to deform the 2 foot secondary mirror and change its shape to compensate for everything from wind blowing against the telescope to atmospheric aberations. It is purported to provide 3 times the resolution of the Hubble telescope. (And you don't have to go into space to fix it."
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A Ground-Based Scope That Flexes For Better Focus More

A Ground-Based Scope That Flexes For Better Focus

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  • hubble is not obsolete yet (Score:5, Informative)

    by u19925 ( 613350 ) on Wednesday February 12, 2003 @03:43PM (#5289826)
    b4 u think hst is obsolete, note that the adaptive optics can only compensate the waveform deformation in a very narrow field. this is good to study binary stars, quasars, galactic neclei etc which are essentially tiny dots in the sky. hst has no waveform deformation at all, so it can use wide field imaging with full resolution; something which is not yet possible with adaptive optics. also adaptive optics requires that either the object is very bright itself or it has some bright objects very close by. hst has no such limitations.

    • Re:hubble is not obsolete yet (Score:5, Insightful)

      by PD ( 9577 ) <slashdotlinux@pdrap.org> on Wednesday February 12, 2003 @04:01PM (#5289951) Homepage Journal
      Plus, time on big scopes is limited and there's a huge demand. Even the Palomar 200 inch scope, with optics that aren't as good as what we'd make today, keeps a full schedule of research. And that thing has to be 70 years old or something close to that. Big research scopes never become obsolete in the sense that nobody wants to use them.

    • Re:hubble is not obsolete yet (Score:4, Informative)

      by suitti ( 447395 ) on Wednesday February 12, 2003 @05:22PM (#5290613) Homepage
      One can make an artificial guide star with a laser. This has already been done. And, the article talks about it.

      6.5 meter main mirror, .64 secondary deformable mirror. This is a big scope. Polomar is 5 meters.

      They hope to image extrasolar planets, for example, to get spectra. HST has already gotten spectra for an extrasolar planet - even though it has not imaged such an object. Step one: get the spectra for the parent star, step two: get the spectra during a transit, step three: compare.

      I'm not sure why they think that a deformable secondary is better than AO afterwords.

      Adaptive Optics are available for the ameteur astronomer. For example, SBIG [sbig.com]

      • Re:hubble is not obsolete yet (Score:5, Informative)

        by Doctor Fishboy ( 120462 ) on Wednesday February 12, 2003 @06:45PM (#5291300)
        > I'm not sure why they think that a deformable secondary is better than AO afterwords.

        Two words: warm surfaces.

        All other AO systems have ~20 warm reflecting surfaces that add up to a high near-IR background, and this severely limits your view of the IR universe. This system adds no additional surfaces into the telescope, and so its IR background is much lower.

        Dr Fish
    • Well this doesn't quite seem like news, not even cutting edge ... currently their are plans in the work to build CELT, a 30 meter telescope with a deformable secondary mirror, while using a laser guide star.

      The laser guide star allows us to view the dark portions of space, where as the UofA system requires that the object be near a natural guide star (namely a star big enough and bright enough that we know where it is supposed to be before it is deformed by the atmospher)

      Land based telescopes are great, and the only reason for space-based ones is to collect the wavelengths of light that are filtered out by the atmosphere. But getting a telescope as large as the 30-meter CELT into space is a big challenge ... however there is a project at Lawrence Livermore National Labs where they have a 15ft. mirror that folds up to the size of a hat box. They brought in an origami expert to figure out how to do this. (Sorry I don't have a URL, I just have a print copy of the monthly LLNL newsletter)

      Anyways, the UofA telescope isn't really news ... it really needs a Laser guidestar ... but there is only working Laser guidestar ... which is at Mount Hamilton
    • Can we also say, "clouds" :)

  • --same tech (Score:2, Interesting)

    by zogger ( 617870 )
    ...same/similar mirror flexing tech they are using for the AirBorneLaser weapon, if I recall correctly.

  • Good, God. The feedback required. (Score:5, Interesting)

    by robslimo ( 587196 ) on Wednesday February 12, 2003 @04:29PM (#5290143) Homepage Journal
    Wow!

    As a person who's spent about 15 years working with closed-loop controls in computer systems, my mind boggles at the thought of the quantity and variety of feedback devices required to pull this off.

    Accelerometers and strain transducers for wind forces, ground vibration and thermal effects on structures at the very least (and multitudes of them, all calibrated with respect to their location, etc). What I'm really having trouble with is how they are managing the thermal and atmospheric compensations.

    OTOH, this is an acedemic project and the statement "we have the *potential* to get images that are three times sharper than the Hubble" (my emphesis added) from the article doesn't inspire great confidence in what they may *really* have.

    Anyway, I'm off to look for answers at this [arizona.edu] link to the Center for Astronomical Adaptive Optics at the University of Arizona, the folks doing this work.

    • Accelerometers and strain transducers for wind forces, ground vibration and thermal effects on structures at the very least (and multitudes of them, all calibrated with respect to their location, etc). What I'm really having trouble with is how they are managing the thermal and atmospheric compensations.

      AIUI, based on reading about other telescope plans, but I think it applies here, they don't try and measure and pre-compensate for distorting influences, rather they use the image to determine what the current distortion is, and compensate for it.

      Of course, to do this you need to have something in the image which you know how it should look (bad grammar there). Eg you arange your picture to contain a point source like a distant bright star and the nebula you are interested in. Then you twiddle the optics until the star is as close to a stationary point as you can get, which should have the effect of making the nebula clear.

      If you have used an auto-focus camera and found that some things are hard for it to focus on, you may have used the same trick, pick somethig near what you want in focus, focus on that and then take your picture. Imagine doing that gazillions of times per second.

      I remember reading a proposal to put optical targets in orbit to help with this process. That way they wouldn't be limited by the need to find a bright point source near what they are interested in.

      • With this kind of tweaking process, it seems possible that they might "tweak" out an optical effect that they should've kept, on the grounds that it wasn't something they were expecting.
        • With this kind of tweaking process, it seems possible that they might "tweak" out an optical effect that they should've kept, on the grounds that it wasn't something they were expecting.

          Depends on the distortion really. For instance if your calibration star is jittering around the image, you can be pretty sure it is due to atmospheric distortion, not to the star leaping light years in milliseconds.

    • They don't actually get the feedback from measuring the strain and deformation of the telescope's various parts - they simply look at the image the telescope is producing, and deform the lens based purely on that. This requires that some calibrated known object is present in the telescope field-of-view, known as a 'guide star'. As noted elsewhere in these comments, shining lasers into the ionosphere are often used as artificial guide stars.

  • What's so great about this telescope? (Score:4, Interesting)

    by bill_mcgonigle ( 4333 ) on Wednesday February 12, 2003 @04:36PM (#5290206) Homepage Journal
    And that's not sarcasm. I couldn't figure out from the article what's really new and great about this particular telescope. Comparing it to one I've read about, the Starfire [af.mil]:

    this vs. Starfire:

    The 40-kHz closed-loop adaptive optics (AO) system adjusted the position of 336 points 941 actuator adaptive optics system on its 640-mm (2.1-foot) 3.5 Meter deformable mirror 550 times per second

    "This is the first time that anybody has done adaptive optics with a mirror that is an integral part of the telescope itself" said Lloyd-Hart. Primary mirror has actuators ...winds of up to 30 mph had no effect on the final image. "Closing the feedback loop is something that nobody else in the world has -- feedback enables us to make our adjustments very, very precisely, because of our constant stream of position feedback," said Lloyd-Hart. Wind buffeting is reduced by the telescope's very stiff structure and high-torque motors and by angular acceleration sensors which control fast-steering mirrors designed to optically cancel out wind induced jitter.

    I'm just a layman who likes reading about telescope technology, but it sure looks like they're making claims of being first when they aren't. Still, the Air Force is funding this, and they have a telescope that can image a basketball at a thousand miles, so there's obviously something good here.

    Is it the 550Hz sampling rate? Maybe it's the first one available to civilian astronomers? Does anybody know?
    • "Is it the 550Hz sampling rate?" No. It's the 40KHz sample rate and 550Hz update rate (updating the actuators). I imagine they do some heavy shit math on the 40KHz data between each update of the outputs.

      The mil scope has active [vibration only?] cancellation where the civvy model has a unique on-scope mirror which is reshaped to compensate for vibration and atmospheric conditions. BTW, the link you cite says the mirror for the mil job was made by the U of A... I think they've one-upped the air force.

    • Reasons why this system is better... (Score:5, Informative)

      by Doctor Fishboy ( 120462 ) on Wednesday February 12, 2003 @06:18PM (#5291100)
      Heh, a /. article that covers something I worked on!

      > but it sure looks like they're making claims of being first when they aren't.

      This *is* a first because the deformable mirror *is* the secondary mirror, which all modern reflecting telescopes have nowadays.

      The Starfire Optical Range (SOR) telescope and all other adaptive optic (AO) systems use about 10 to 20 additional reflections between the sky and the detector to do the AO correction - it may be a 3.5 meter telescope, but it's more like a 1 meter telescope in light-gathering power after 20 bounces for light loss are taken into account. The SOR telescope was also optimised for taking high resolution picutres of fast moving objects in low earth orbit *think spy satellites*

      The other bonus is that the new system is *excellent* for taking near infra-red pictures of the night sky, and a lot of recent astronomy is driven by a need for a good AO system in this regime (about 1 to 10 microns).

      Dr Fish
      • The SOR telescope was also optimised for taking high resolution picutres of fast moving objects in low earth orbit *think spy satellites*

        Ok I am thinking *spy satellites*. Can an adaptive optic system be used to focus beams of energy on an object in LEO rather than image them?

        • In principle, yes. However, the problem has been that the laser heats up the atmosphere as it goes through it, causing "blooming." Essentially the heated gas around the beam has different optical properties than the quiescent gas, then as the atmospheric gases of different temperature start to mix, get a randomn distortion of the beam. Multiply this over the distance you are trying to focus the beam, and it becomes pretty much impossible to focus properly. Now, if you've already got a laser in space....

          At least, that's how it was explained to me...

    • > on its 640-mm (2.1-foot) 3.5 Meter deformable mirror

      You're comparing the wrong things together.

      It's not clear in the article, but the Starfire telescope has a PRIMARY mirror diameter of 3.5m, but the Steward telescope has a diameter of 6.5m.

      The flexible secondary mirror of the Steward is the 640mm number you've compared with the primary mirror of the starfire telescope.

      Hope that clears it up for you!

      Dr Fish
  • Anyone have any good references for an introduction to the techniques used in adaptive optics? I'm looking for something a physics graduate might understand.
  • ... they didnt think of this when they made the mirror for Hubble which was messed up in the first place. :-)

    Note: I'm a UofA Alum, but their screwup on that mirror deserves mention whenever they do something right...

    • Re:Too bad... (Score:5, Informative)

      by Doctor Fishboy ( 120462 ) on Wednesday February 12, 2003 @06:50PM (#5291330)
      > Note: I'm a UofA Alum, but their screwup on that mirror deserves mention whenever they do something right...

      I'm trying to check my history, but wasn't the Hubble mirror made and tested at Perkin Elmer? I don't remember the U of A being involved with Hubble's mirror. It was an error in the test equipment at Elmer that led to the spherical aberration being put into the mirror...

      You may be thinking of the NICMOS camera, which the U of A was involved with, and that had a problem with cryogens boiling off too rapidly, but that was corrected a while ago...

      Dr Fish
    • I know your comment is tongue-in-cheek, but I'll add anyway that the actuator and thin mirror technologies weren't available in the early 80's, and you wouldn't have been able to put up the processing power (then, or even now I suspect).

      These kind of things are in the future plans for NASA from the Jack Webb Space Telescope on out (sorry, I don't have the time to dredge up some links).

  • Since our primary mirror is three times bigger than the Hubble, we have the potential to get images that are three times sharper than the Hubble.>>
    Can you say atmospheric diffraction? As long as you keep that badboy on the ground it's going to be lacking something. Pretty pictures, yes, empirical data, no.
  • Yeah, but all of the light pollution and atmospheric diffraction etc makes for worse images than the Hubble can provide. A 1500dpi scan of a fuzzy photo is no better than a 300dpi scan.
    • However, better resolution doesn't have to be for imaging. There are a handful of optical interferometer projects (ground and proposed space) that have amazing resolution and are used for very narrow field of view imaging as well as astrometry. In this case, more resolution == better telescope (at least for its intended application). This is why, for instance, NASA [nasa.gov] has the JWST [nasa.gov] as well as SIM [nasa.gov]. Different scopes for different folks.

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