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World's Largest Telescope Begins Production
JohnnyNapalm writes "The Aggie Daily News is reporting today that the first mirrors have been cast for the world's largest telescope. The result of cooperation from some of the foremost institutions in education and science in the nation, the Giant Magellan Telescope stands to operate at a resolution 10 times larger than the Hubble. The project, set to be constructed in Chile, is slated for completion in 2016."
The House of Representatives recently approved funding to service the Hubble.
That's true, but it's simple economics, unfortunately. To build an earthbased telescope, it is cheaper by a factor far outweighing the costs of hoisting the equivalent mass into orbit. On a side note, most ground telescopes utilize correctional algorithms to help with atmospheric distortion. Sure, stars sparkle down here, and dont up there, but NASA's purse is a bit light lately.
Photos
Largest optical, perhaps...until OWL!
"With a diameter of 100 meter, OWL [Overwhelmingly Large Telescope] will combine unrivalled light gathering power with the ability to resolve details down to a milli-arc second."
Link: OWL
No gods, no demons, and no masters. Secular Humanism!
AFAIK, both schools are public universities. So instead of the United States Government being able to yank the plug, its in the hands of the State of Texas.
Sadly, that still might be advantageous.
It does use adaptive optics. Have a peek at the tech section of the GMT site, here: http://www.gmto.org/tech_overview>
From the aforementioned link: The GMT secondary mirror is composed of seven thin adaptive shells, with each segment mapping to a single primary mirror segment. The adaptive secondary will provide diffraction-limited performance over modest fields of view and ground-layer adaptive optics over a field of ten to twenty arcminutes in diameter.
-scott
My other sig is a Glock
And of course the GMT is being built as a single scope with one focus, while things like the VLT, Keck and LBT use interferometry to get sharper images.
(And adaptive optics! I want telescopes with frickin' laser beams strapped to their heads!)
Village idiot in some extremely smart villages.
I agree, it'll be easier to build (although OWL will use multiple segments/single focus).
But...when I read "..milli-arcsecond" resolution (in the optical!) on the OWL site, in spite of its competitors, my knees got weak, my toes curled...And I'm a grown man.
I and the guy who teaches the class I TA for recently had students calculate how large a primary would be necessary to read a homework page on the moon, from Earth. (assuming, of course, diffraction-limited seeing...hah!). Needless to say, even OWL wouldn't cut the mustard. But it'll be way, way cool, even without laser beams.
Oh - and speaking of laser beams and telescopes: Apache Point Apollo Laser. (I've been down to see this...very cool!)
No gods, no demons, and no masters. Secular Humanism!
We play with lasers over on Mauna Kea, too... like this nice 20-watt sodium dye one. Which, for topicality, is located at the world's current largest optical telescope...
Village idiot in some extremely smart villages.
For something a little closer to completion than 2016, check out the Southern African Large Telescope. Scheduled to open in November, and will be the biggest optical telescope in the southern hemisphere.
Regards,
-Jeremy
>not actually deforming the mirror segments
IIRC The mirror segments were deformed during construction. The mirror segments need to be ground to the correct shape (with a pretty tight definition of correct). I belive they were deformed in such a way that the actual shape ground was an easy one to do. When the mirrors were released they sprung back to their original overall shape but with the surface ground to what was needed for the final mirror. Neat way of getting around the problem.
They intend to use adaptive optics to compensate for the effects of the atmosphere, though if you read the science case they haven't quite decided on how to do this yet.
For those of you not familiar with why astronomers would place (frickin') lasers onto telescopes, there are multiple reasons.
The primary reason is to provide a "fake star" that can be monitored for distortion, which helps adaptive optics systems counteract atmospheric distortion in the final telescope image/data. In other words, it helps remove the "twinkle" caused by the atmosphere.
The laser at Apache Point, as well as at other locations (see previous message), is used to measure the distance to the moon (which is useful in, among other things, studies looking at the accuracy of general relativity).
The Apache Point laser is capable of measuring the distance to the moon to a millimeter using this device. (think about it: at a telescope, up on a mountain around 10,000 feet, there's probably more 'flex' in the mountain itself!).
No gods, no demons, and no masters. Secular Humanism!
I can't think of many non-military organisations which have bigger budgets
I can. 2005 Numbers:
Department of Health & Human Services: 584B
Department of Education: 56.5B
Department of Veterans Affairs: 32.5B
Department of Housing & Urban Development: 32B
Department of Homeland Security: 29B
Department of State: 27.5B
Department of Energy: 23.8B
Department of Agriculture: 21.4B
Department of Justice: 20.2B
NASA: 16.1B
Cheaper Departments include: Treasury, Transportation, Labor, Interior, Drug Administration, EPA, and Commerce. They generally run 8-15 billion each.
Source: Washington Post
I don't read AC A human right
It's one piece of glass, with a single, smooth surface on the front, 8.4 m in diameter. The hexagonal "pieces" are holes on the backside. It basically looks like a big honeycomb. This design gives you great stiffness and strength, with only 20% the weight that a solid mirror would have.
Liberal (adj.): Free from bigotry; open to progress; tolerant of others.
The Steward Observatory Mirror Lab had an open house yesterday for observatory personnel, which I attended.
The spin-cast oven is huge. In these pictures, you only see the top portion of it, it actually fills the floor below as well. I believe this is the only large spin-cast mirror facility in the world. The idea behind spin-casting is that, by spinning the molten glass as it is slowly cooled, you automatically get a paraboloid top surface. This makes the final shaping of the mirror much easier, since the first-order shape is already there.
Actually, in the case of the GMT, it will use seven mirrors, six of which are off-axis. The off-axis mirrors will obviously have a more complicated surface than a typical on-axis paraboloid. The mirror being cast now is an off-axis mirror; it is a proof-of-concept that they can grind an eight-meter chunk of glass to an off-axis paraboloid shape with a surface RMS of 20 nanometers (!).
In a few months when the mirror has cooled and solidified, it will be removed from the oven, cleaned, ground, and eventually, polished. The stress-lap polisher is very impressive. It has a network of stress actuators above it, which can dynamically change the shape of the polisher's surface as it travels across the mirror.
It's interesting that the "Aggie Daily News" was chosen as the linked story, which makes it sound like UT Austin and Texas A&M are the major players in the GMT, along with a handful of other, unnamed institutions. In fact, the Carnegie Institute is the impetus behind the project, and the U of Arizona is providing the mirrors. I think this UA News article is much more informative.
Liberal (adj.): Free from bigotry; open to progress; tolerant of others.
The largest dimension on the Apollo landers is 9.07 m, diagonally between the landing legs (it's a 21 foot square). The moon's closest approach to the earth is 363,104 km. Divide those two numbers, and you get the angular size in radians: 2.36e-8 radians, or 0.00487 arcsecond.
With a 27m diameter, the diffraction limit on telescope resolution is 10^8 cycles/radian. So if there were no atmosphere, it would be barely possible.
With an atmosphere, there are problems. A typical good seeing limit is 1 arcsecond (corresponding to 1864 m on the moon). The best on the planet (Dome C, in the middle of Antarctica, at a very good time) is 0.2 arcseconds (373 m on the moon).
It's possible for adaptive optics to get good enough to allow that much correction, but it's going to be difficult.
Hoewever, if you want optical evidence for the Apollo landings, just shine a bright light at the moon and look for the retroreflector arrays the astronauts left there. That experiment has been done a zillion times in the last few decades.
You just need a very, very accurately aligned surface. For moderate sizes, this is easiest to achieve by just making the whole mirror a solid piece. As the mirror gets larger, the stiffness needed to hold its shape to within a fraction of a wavelength of light gets more and more difficult to achieve. Above 5m (the 200 inch Hale telescope), you have to change to active supports, where you measure and compensate for gravity sag rather than just trying to be stiff enough.
There's still a tradeoff, though. The mirror needs to be stiff enough to hold its shape between support points. A thinner mirror weighs less, but requires more active supports.
Past 8m (27 ft), it's just not feasible to build or transport a single mirror, so folks cut it into pieces, effectively sending the mirror thickness to zero in some places. This further complicates the active support system, but it's possible.
The gaps between mirror segments are no different than the shadow of the spider supporting the secondary mirror; they're just places you don't get any light from. They don't pose any particular imaging problem.
You can cut the round mirrors into hexes and pack them closer; this reduces the overall size of the telescope and makes it easier to build. But after considering the posibilities, the GMT folk decided to leave the mirrors round and accept larger gaps. This actually makes a slightly better telescope. No cutting means more mirror area. Wider edge-to-edge gives a better diffraction limit on resolution. And it turns out the round edges produce a nicer point spread function.
"Hubble Class" is a dangerous phrase. The thing that made Hubble expensive wasn't that it was a certain size - it was that it, like every terrestrial telescope, allowed for upgrades, switching in new instruments, and so on.
All other space telescopes that I'm aware of - using the forthcoming James Webb Space Telescope as an example - don't do that. They're non-upgradeable, with "no user-serviceable parts." They're not dependent on the Shuttle in any way. In some cases, like Webb, they're not even destined for orbit, but for a Lagrange point. (Others, like the current Spitzer scope, are in orbit, of course.) That makes them a lot cheaper than Hubble - but it also gives them a shorter mission life, less flexibility, and no option for upgrades down the road.
As for Hubble being the "first ever optical space telescope" (which it wasn't, if you count the OAO UV observatories NASA launched 20-25 years earlier) and Keck being "the 1,000th ever research class terrestrial telescopes" - keep in mind that the Kecks used that segmented mirror design, which was completely untested and radical at the time they came up with it, and had a 66% larger diameter than anything else out there (the Soviet BTA 6-meter was the largest out there before it). So while Keck may have been the 1,000th ever research class terrestrial telescopes, they were quite unlike those that came before them.
A single shuttle mission costs more than most of the largest optical telescopes in the world today.
Village idiot in some extremely smart villages.