World's Largest Telescope Begins With A Spin

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zavvy

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<b>World's Largest Telescope Begins With A Spin</b><br /><br />LINK<br /><br />Expert mirror makers are gearing up to cast the first component of what is planned be the world's largest telescope. When completed, its seven 8.4-metre mirrors will more than quadruple the power of today's best observatories.<br /><br />Like other multiple-dish instruments, such as the four 8-metre dishes of the Very Large Telescope in Chile, the planned Giant Magellan Telescope will take advantage of its expansive surface area to boost its sensitivity to light. <br /><br />But unlike the VLT, the GMT will arrange its multiple mirrors into a single "super dish” - with one mirror at the centre and the other six curved around it like petals. Such "off-axis" mirrors have never been made as large as this. <br /><br />The design makes the telescope's vision keener than it would be if all seven mirrors remained separate. It will have 10 times the resolving power of the Hubble Space Telescope.<br /><br />"The double advantage is that by making them into a single surface, you get a sharper image as well as collecting more light," says Roger Angel, director of the mirror lab at Steward Observatory in Tucson, Arizona, US.<br /><br />Grind and polish<br />Steward Observatory is home to the world's only spinning glassmaking furnace, which is now going through final warm-up before casting the first of the GMT's seven mirrors. The spinning oven uses centrifugal force to give the mirror its initial concave shape, saving greatly on the cost of glass and the time and work needed to grind and polish the mirror into its final form.<br /><br />Construction of the mirror's mould has just been completed, and the oven is now undergoing a pre-firing before technicians carefully load it with 18,000 kilograms of borosilicate glass made from sand gathered on Florida's Gulf coast. <br /><br />The glass will be heated for about a week, starting around 18
 
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petepan

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I was telling a friend about the size of this thing. I was able to show him a pic, just for reference.<br /><br />This is gunna be awesome <img src="/images/icons/smile.gif" /><br /><br /> <br /><br /><i>EDIT: forgot to quote this info</i><br /><br />Workers completing the mold of the 8.4 metre mirror for the Giant Magellan Telescope mirror. Image credit: Lori Stiles/UA
 
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bushuser

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Should I assume each mirror has adaptive optics? Otherwise, it seems all that resolving power would be smudged by the turbulent atmosphere. I thought that was Hubble's big advantage, since its mirror is only 2.4 meters diameter.
 
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rogers_buck

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I know some of the history on this one!!!<br /><br />Roger Angel was my advisor and professor years ago. Back then, the MMT (affectionately called the 6 pack because of its 6 70" mirrors) was the first new technology telescope of its kind. The UofA kicked in with SAO to build this brainchild of Aden Meinel.<br /><br />Back then, I was Meinel's RA student. One of the things we were working on was spin casting of large parabolic molds for explosive forming mirrors. I made an 8-inch mirror overnight from epoxy by spin-casting and aluminized it and tested it. Spin-casting wasn't new, Nat Carlton of SAO had made spin cast mirrors a few years earlier and the large Hg telescope was either in the works or in operation in NM (can't recall which). Meinel showed Angel the mirror that I had made and he was impressed with the technique. Angel went on to build a small furnace in his back yard to experiment with spin-casting, but with glass.<br /><br />Years later, one of Angel's big spin-cast mirrors (6 meter I think) was used to replace the six mirrors of the MMT with a single mirror. So, it sounds like things have come full circle. Now he is building a MMT with his giant spin-cast mirrors....
 
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rogers_buck

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Last time I spoke with Roger Angel was about a decade ago. At the time, he was working with neural networks for adaptive optics control. It may well be the case that more conventional linear controls are to be used instead, however. Seems like that is usually what winds up happening with this type of system. Maybe someone else here has some more up-to-date info on this?<br /><br />The MMT (6, 70" mirrors) had an adaptive optics system. The ALPHA version consisted of a coaxial laser with a 6 way beam splitter. The beams were directed into the primary mirrors after bouncing off convex mirrors to serve the purpose of artificial stars. A CID (Charge Injection Device) camera (160x160 as I recall) was then used to close the loop with the servo system. Something interesting was discovered when the active optics system wasn't ready for primetime and the telescope used with manual adjustments. Since the telescope was so mechanically stable the active optics were redundant. I think it ran that way for years. Of course the new giant won't be so lucky...
 
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zavvy

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<font color="yellow">I know some of the history on this one!!! </font><br /><br />Wow! Great story, rogers_buck! <br /><br />Steve.. the site is on a <i>what</i>..?? <br /><br /><img src="/images/icons/shocked.gif" />
 
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rogers_buck

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The choice of the southern hemisphere was likely governed by coverage of astronomical targets. As for earthquakes - any active optics worth their salt could compensate for a 9.0. (-;<br /><br />The spin-cast process is used to eliminate a few tons of glass that would normally have to be ground away in the usual process. If grinding were used, it would take an eternity to cool the mirror for each step and add an impractical amount of time just for coarse removal of the unwanted glass. Final polish and figuring of a spin-cast mirror is performed in the usual way.<br /><br />For off axis spin-casting all they need to do is offset the center point of the rotating mold. Sounds easy except for the tons of molten glass...<br /><br />Telescopes like the McMath 4-meter on KPNO had mirrors made of cermet. This ceramic glass was used because of its low coef of expansion. Mienel attended a design meeting for a large new telescope once where the use of a cermet mirror was being debated. He quietly scribbled on his paper a calculation and then interrupted the discussion to point out that the mirror would remain in perfect figure for the planned temperature differentials, but the steel telescope would have contorted some embarassingly unserviceable amount... Cermet was dropped from the planning and an air conditioner was added.
 
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vogon13

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Jes' curious, is the shape of the spinning molten glass in cross section a parabola or a catenary? (or some other shape?)<br /><br />Had contemplated putting old turntable in freezer and making an ice disk and then measuring it, but decided it would be easier to ask and see if anyone knew.<br /><br />Would a sheet of plastic over a partially evacuated cylinder have the same shape?<br /><br />Would the shape of that plastic sheet stay the same (class of curve) if the depression was filled with water ?<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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rogers_buck

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It's a parabola.<br /><br />The tangent to the surface is the centrifugal force divided by the gravitational force. Integrate that and you have a parabolic solution for y.
 
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vogon13

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thanx!<br /><br />Math aspect of all that is scary, but I accept fully the conclusion.<br /><br /><img src="/images/icons/wink.gif" /><br /><br />Guess that really cuts down on grinding of glass blanks then. I wonder how far out in the infrared you would have to go to not have to grind a spin cast mirror at all.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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