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Science Technology

"Perfect" Mirrors Cast For LSST 114

Posted by kdawson
from the billion-pixels-of-goodness dept.
eldavojohn writes "The Large Synoptic Survey Telescope (which was partially funded by Gates & Co.) announced a world record casting for its single-piece primary and tertiary mirror blanks, cast at the University of Arizona. From the announcement: 'The Mirror Lab team opened the furnace for a close-up look at the cooled 51,900-pound mirror blank, which consists of an outer 27.5-foot diameter (8.4-meter) primary mirror and an inner 16.5-foot (5-meter) third mirror cast in one mold. It is the first time a combined primary and tertiary mirror has been produced on such a large scale.'"
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"Perfect" Mirrors Cast For LSST

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  • Re:Apparently... (Score:5, Informative)

    by Anonymous Coward on Tuesday September 02, 2008 @08:51PM (#24853165)

    Apparently it was so awesome, they just skipped the secondary mirror and went straight to tertiary. :)

    The optical design is somewhat unusual as it has
    three mirrors, but this is required to get a very
    large field of view (with diameter equal to 7 full Moons).
    The secondary mirror will be made separately.
    If you are interested in more details
    about LSST, please take a look at our website,
    http://www.lsst.org,
    and a review paper
    http://lanl.arxiv.org/abs/0805.2366

          Cheers,
          Zeljko

  • by hey! (33014) on Tuesday September 02, 2008 @09:04PM (#24853275) Homepage Journal

    In optics, you get to the point where further "perfection" doesn't give you any pratical benefit. That is being "diffraction limited". Diffraction limited optics are for practical purposes as "perfect" as you can get.

    For a telescope operating through the Earth's atmosphere, you run out of marginal advantage before you reach diffraction limitation. Therefore for such a system, unless special techniques such as adaptive optics are used, practical "perfection" is considerably lower.

    I don't know much about the LSST, except that it is a fast (short focal length relative to aperture) optical system. Such systems are much more difficult to get right. Long focal lengths are much more forgiving. Therefore to reach practical perfection in such an aggressive design is quite an achievement. Of course, we aren't there yet. There's three absolutely huge surfaces to grind to very price specifications. But simply casting a blank this size is a huge technical challenge. The amount of heat energy in twenty six tons of molten glass is mind-boggling. Getting it cast into a shape that can be ground and polished into an optical mirror is an engineering tour de force in itself.

  • Re:Apparently... (Score:5, Informative)

    by againjj (1132651) on Tuesday September 02, 2008 @09:35PM (#24853579)

    Mod parent up (despite limited details).

    The design is called Paul-Baker/Mersenne-Schmidt.

    Page on the telescope design: http://www.lssto.org/Science/lsst_baseline.shtml [lssto.org]
    Wikipedia article on telescope: http://en.wikipedia.org/wiki/Large_Synoptic_Survey_Telescope [wikipedia.org]
    Wikipedia article section on the Mersenne-Schmidt design: http://en.wikipedia.org/wiki/Schmidt_camera#Mersenne-Schmidt [wikipedia.org]
    Paper on the Mersenne-Schmidt design: http://adsabs.harvard.edu/abs/1984MNRAS.210..597W [harvard.edu]

  • by Trogre (513942) on Tuesday September 02, 2008 @10:09PM (#24853873) Homepage

    I'm not sure "good enough" is the right term. The point is that the mirror produces results utterly indistinguishable from a mathematically perfect surface.

    Nothing to do with settling for "good enough" which usually implies a compromise has been made somewhere.

    You could say it was practically perfect in every way. I'll go stand outside now.

  • Mirror and Camera (Score:5, Informative)

    by stewardwildcat (1009811) on Tuesday September 02, 2008 @11:41PM (#24854659)
    I am an astronomer at the UA and the mirror is a major feat of engineering. It will be the first telescope to have the tertiary and the primary mirror on the same piece of glass. They will have to grind both parts to be perfectly aligned (point to the same place) as well as make the transition area as small as possible. The secondary mirror is a doughnut shape that will be placed above the primary and will have the $100M camera behind it. The camera itself will be the size of a small car and will be as stated before a 3.2 Gigapixel ccd. It will have 200+ 4k by 4k CCD chips that will be read out in 2 seconds. This coupled with the fact it will image the night sky in 5 colors every week will lead to petabytes of data by the programs terminus. Its basically the coolest telescope that will ever be built. ESPECIALLY since the data is set to be public (for US residents) the moment it is processed each morning.
  • by Anonymous Coward on Wednesday September 03, 2008 @12:09AM (#24854813)

    Casting 8-meter-class blanks simply isn't that uncommon any more. The Large Binocular Telescope has a pair of 8.4-meter primaries; Subaru has an 8.3-meter; VLT has four 8.2-meter, Gemini North and South each have an 8.1-meter. Oh, and the Giant Magellan Telescope [gmto.org] is planned to have seven 8.4-meter mirrors.

    The LSST is unusual in that its light path is more "folded", hitting 3 mirror surfaces on the way to its primary camera, which means that relatively run-of-the-mill 8-meter-class blank has to be ground pretty uniquely. (And I wish them the best of luck with the process.)

    Also, its secondary mirror is absofreakinglutely huge, at 5 meters. To put this in context, just ten years ago there was only one operational telescope in the whole world with a primary mirror larger than 5 meters.

    And f/1.25 is crazy fast, yes. The newest, fastest survey scopes out there right now are VISTA at f/3.25 and Pan-STARRS PS1 at f/4. SDSS is f/5, and VLT is f/5.5.

    So there you have it - what's really cool about LSST, from a guy who drives a boring old f/10 2.2-meter. ;)

  • by Shag (3737) on Wednesday September 03, 2008 @12:11AM (#24854831) Homepage

    So there you have it - what's really cool about LSST, from a guy who drives a boring old f/10 2.2-meter. ;)

    ...and who absent-mindedly checked the "Post Anonymously" box for no reason. Whoops.

  • Re:why not an array? (Score:5, Informative)

    by Nyeerrmm (940927) on Wednesday September 03, 2008 @12:17AM (#24854893)

    There's a difference between a segmented or even sparse primary mirror and an interferometer.

    A telescope with a segmented mirror works almost exactly the same way as a normal telescope, except its easier to manufacture mirrors. Of course, this is with the cost of making it harder to keep aligned, and introduce unnecessary complexity for a small mirror, but as the sizes grow it becomes more and more cost effective to segment.

    A sparse mirror with a well designed layout (say a Golay array) will be very effective also as a traditional telescope. The array is designed to gather all the spatial frequencies (think of a telescope as an analog Fourier transform) with as few elements as possible. Thus, though gathering less light, it will create an image of the same resolution. Of course less light leads to lower SNRs which can be tricky and is why you don't see too many sparse systems right now.

    An interferometer, while conceptually similar to a sparse aperture system, only measures a single frequency component at a time, by taking the light from two distant telescopes and interfering them to determine the "fringes" (Young's experiment) which measure how similar the light beams are. It is thus very precise, but also very limited. Given enough time and patience you could move the relative positions of the telescopes to fill out the Fourier transform, but this is usually not very practical given that alignments need to be maintened within 10s of nanometers.

  • Re:why not an array? (Score:5, Informative)

    by edremy (36408) on Wednesday September 03, 2008 @01:45AM (#24855357) Journal
    LSST isn't interesting because of the mirror diameter, it's interesting because of its incredibly wide field of view and amazingly fast optics. This thing has a field of view of almost 10 square degrees and can image down to ~24th magnitude every 15 seconds. Nothing else built or planned even comes close. PAN-STARRS4 will be the nearest thing to LSST and it has an etendue* that's something like 1/6th of LSST, although the PAN-STARRS people like to point out it's also something like a 5th of the cost of the LSST. (* a measure of mirror diameter*field of view. Bigger is better for survey telescopes)

    The UT system isn't even the same idea- the main mirror can't even be moved in elevation and doesn't cover the entire sky- it only sees 70% of it. Hobby-Eberly is a spectroscope, designed to look at specific targets for a long time to get the spectrum of the target. LSST is a survey telescope- it's going to scan the visible sky every 3 days in multiple wavelengths, so you have to have an entirely different grade of mount, support structure and drive system. As any amateur astronomer will tell you, cheaping out on the mount will save you quite a few bucks. :^) (Although looking over the Hobby-Eberly, they did some really neat stuff with the mount to get it to track.)

    Entirely different missions, different optics, different mounts, etc etc.

  • by bdeclerc (129522) on Wednesday September 03, 2008 @05:49AM (#24856439) Homepage
    Yes, it's the same. A shorter focal ratio ("fast") allows for a larger field of view with the same size CCD-chip. The "side-effect" of this is that a larger bit of sky falls onto the individual pixels, which means you sacrifice resolving power for sensitivity.
  • by bdeclerc (129522) on Wednesday September 03, 2008 @06:41AM (#24856711) Homepage

    A telescope mirror needs a number of special properties, from rigidity and weight, but also thermal stability and the ability to polish it efficiently.

    For nearly 50 years the largest mirror was the 5 meter Hale telescope, but in the late '80s materials science and casting techniques had evolved to the point where we could reliably cast larger, lighter telescope mirrors, and computing power to the point where active suspension of thinner mirrors is possible.

    However, this doesn't mean we can create weightless mirrors, and an 8.4m mirror with a short focal length and two different surfaces still requires quite a bit of internal strength. Glass still has a higher density than water.

    An 8.4m mirror has a surface area of 220 square meters, even assuming the density of water (1000 kg/m3) 25 tons corresponds to a thickness of only about 12 cm, or less than 5 inches, which is very very thin, and as I said, glass has a density higher than water, so the actual thickness would be substantially less.

  • by budgenator (254554) on Wednesday September 03, 2008 @07:15AM (#24856885) Journal

    the Hubble mirror is extremely accurate, unfortunately the testing mechanism, the null corrector wasn't, so the Hubble mirror was ground and figured almost perfectly wrong.

  • by budgenator (254554) on Wednesday September 03, 2008 @07:21AM (#24856905) Journal

    At this point, perfection is measured by the glass being approximately the correct shape and without air bubbles or strains being too close together or the surface. If you watch the video, someone was actually walking on the mirror, something that couldn't be done after the blank was ground and figured to an accuracy of a few millionths of an inch.

  • Re:why not an array? (Score:3, Informative)

    by budgenator (254554) on Wednesday September 03, 2008 @07:29AM (#24856959) Journal

    this is a fast wide-field telescope, it's designed to be wide-angled and low magnification, most other scopes are narrow-field and high magnification. This one will take pictures of the whole observable sky over and over so changes over time can be easily seen, hell they could even make time-lapse movies!

  • by Muad'Dave (255648) on Wednesday September 03, 2008 @08:45AM (#24857489) Homepage
    It's not that the surface is perfect to the limit of our ability to measure, it's that the performance of the telescope _system_ is constrained by something other than the shape of the mirror (diffraction-limited). The mirror is "utterly indistinguishable" from perfect because any more perfection out of the mirror will not increase the _system's_ performance. In other words, the telescope's performance would not be enhanced at all if the mirror were replaced with a mathematically perfect one.

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