Slashdot Mirror

To me, this is no surprise. The science case for the GMT is a drool-worthy cornucopia of astrophysics, including formation of stars and planetary systems, properties of exoplanets (including their atmospheres), chemical evolution in stellar populations, dark matter and dark energy (including synergy with the LSST), galaxy formation and evolution, and the first light and reionization of the universe. The potential for observing non-equilibrium chemistry in extrasolar planetary atmospheres is pretty darned exciting with consequences that could reach beyond astrophysics to religion, philosophy, and policy.

It seems the future of astronomy is bright.

For those who don't want to visit Forbes, a site with a history of malware in their ads but insists that visitors disable ad blockers (and script blockers like Noscript)...

Here are a few links with more information than you'll get from Ethan's article, plus they don't require disabling ad blockers:
http://www.space.com/31079-giant-magellan-telescope-groundbreaking-travelogue.html
http://gizmodo.com/the-giant-magellan-telescopes-fourth-mirror-melting-is-1736954773
http://www.gmto.org/resources/

The technology is pretty damn cool, especially the adaptive optics to deal with atmospheric turbulence. It's worth a read, especially when you don't have to try to visit Forbes.

I really wish the Slashdot editors would stop letting this crap through. But because they do, it's a good service to everyone if users can provide alternate links that are better. In this case, there's a hell of a lot of good information on the actual GMTO site.

That's why I skip the articles and just look for the information I'm interested in. Like, hmm, how will this ground-based atmosphere-ridden telescope compare to the Hubble Space Telescope?

From the FAQ on http://www.gmto.org/ ... which is linked,

The GMT will leverage cutting-edge optics technology to combine seven giant mirrors to achieve 10 times the angular resolution of the Hubble Space Telescope in the infrared region of the spectrum.

When coupled with the GMT adaptive optics (AO) system they will produce images sharper than those from the Hubble or Webb Space Telescopes.

And it goes on to explain that the atmospheric turbulence 200 meters up can be measured with lasers, and the one of the mirrors is physically deformed to compensate for the measured distortions. Pretty neat.

Telescopes already use the convention:

Large
Very Large
Giant
OverWhelmingly Large

http://en.wikipedia.org/wiki/European_Extremely_Large_Telescope at 39m http://www.eso.org/public/teles-instr/vlt.html at 4 x 8.2m and my favorite mainly because of it's sheer size http://www.gmto.org/ at 6 x 8.4m

I'm sure one of the first things that you think of when using big pieces of glass is the fact that they'll get dirty. A little bit of googling tells us that the mirrors will be regularly CO2 -cleaned, (basically blasting all dirt off the surface of the mirror) - see section 10.11: http://www.gmto.org/science-conceptu.html . Each mirror will also get recoated every 2 years, to prevent scratches and blemishes.

In addition to the vast distance -- the very closest star to earth is four light-years away

I guess that's what you get for sleeping during the daytime and working at night.

Are you suggesting the 7 mirrors will be all next to one another in a long line with 58 meters of length?
If so, you might want to look here to get a better idea of how it will look like.
And you may want to go here to see it will actually have a 24.5 meter diameter, which is a lot closer to 22 than to 58.

But hey, don't let facts or the possibility of learning something get in your way... every man deserves the chance to call someone else an idiot.

Are you suggesting the 7 mirrors will be all next to one another in a long line with 58 meters of length?
If so, you might want to look here to get a better idea of how it will look like.
And you may want to go here to see it will actually have a 24.5 meter diameter, which is a lot closer to 22 than to 58.

But hey, don't let facts or the possibility of learning something get in your way... every man deserves the chance to call someone else an idiot.

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 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. ;)

Complete bullshit.

Your cost estimates are accurate, but the rest of your argument is total shit. Adaptive optics, WHEN it works (which is rarely, and with difficulty), can approach the angular resolution of HST in a VERY SMALL field of view. You cannot get 0.05 arcsec, diffraction limited images over a wide field of view, that is possible with HST.

"Designing a mirror to withstand a launch vehicle" is a problem that has been solved. And the only two current, viable telescope proposals for telescopes larger than 10m are the Thirty Metre Telescope (TMT) and the Giant Magellan Telescope (GMT). OWL is not a concept that is being taken very seriously...ESO certainly hasn't put its money where its mouth is.

Your final point, about not many lines in that part of the spectrum, belies a complete lack of understanding of what you are talking about. The UV (accessible with STIS, and the Cosmic Origins Spectrograph, which will fly on SM4 in late '08) are so full of lines that they overlap all over the place. See, for example, Morton (2003), ApJS, 149, 205, for a comprehensive list. At low redshift, lines of HI, OI, OVI, CIV, NV, CII, SiII, SII, FeII, NI...all are in the UV, in the STIS band. Furthermore, space is the ONLY place these wavelengths can be observed, because of the atmosphere is opaque to wavelengths shorter than about 3300 angstroms.

With some great techniques for correcting the disturbances our atmosphere creates and a lot of huge (e.g.http://www.gmto.org/) ground based telescope slated for construction, it seems that super expensive space telescope will fall out of favor. I think we def need to continue with the JGW scope though - or at least send something to Lagrange point 2 before china does.

And in other news...
http://www.gmto.org/

I like this visualisation of the size of this massive thing.

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.

Hmmm... the GMT site seems to indicate that the mirrors either are hexagonal or are round but in hexagonal frames of a sort. I don't see how this would result in hexagonal holes, though. In the case of the Keck design, the hole to the cassegrain focus is hexagonal because there's one hexagonal segment "missing" to make the hole - but Keck's segments are only something like 1 meter each. In the case of the GMT, the segments are so large that it's simpler to just have a round hole in the middle of one.

That said, while it does seem like common sense to keep Hubble going, because we obviously need top notch visible light telescopes, I guess we have to ask how good we could do today with that same money using ground based adaptive optics telescopes. For less than the price of one Hubble service mission, we could build the Giant Magellan Telescope.

That said, while it does seem like common sense to keep Hubble going, because we obviously need top notch visible light telescopes, I guess we have to ask how good we could do today with that same money using ground based adaptive optics telescopes. For less than the price of one Hubble service mission, we could build the Giant Magellan Telescope.