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World's largest optical telescope gets construction approval

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April 17, 2013

The Thirty-Meter Telescope (TMT) (Image: Courtesy TMT Observatory Corporation)

The Thirty-Meter Telescope (TMT) (Image: Courtesy TMT Observatory Corporation)

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The world’s largest optical telescope got the go ahead last Friday when the Hawaiian Board of Land and Natural Resources (BLNR) granted a building and operating permit for the Thirty Meter Telescope (TMT) to be sited on a plateau of Mauna Kea on the island of Hawaii. The next-generation telescope, which uses a 30-meter (98 ft) segmented mirror promises to capture images from the near-ultraviolet to the mid-infrared wavelengths with unprecedented clarity.

The green light on construction follows extensive hearings to determine the impact of the new observatory on the area near the volcano’s summit and how the TMT Observatory Corporation would address these issues. The permit process has been held up for two years by a series of objections to building the TMT based on environmental impact, aesthetics and possible interference with native Hawaiian cultural practices, but final approval came at a hearing on April 12 in Hilo, Hawaii, where the TMT was granted a Conservation District Use Permit.

Basic layout of a Ritchey-Chrétien telescope (Image: Tamasflex/Wikimedia)

The TMT is a wide-field, Ritchey-Chretien telescope, where the light is collected by the primary mirror, reflected back to focus on a secondary mirror, and then back again through a hole in the primary mirror to an eyepiece, camera or instrument. The distinguishing feature of the Ritchey-Chretien telescope is that both the primary and secondary mirrors are hyperboloidal. That is, their cross section follows a hyperbolic curve.

The 30-meter size was selected for a number of reasons. One obvious factor is that a larger telescope can capture more light and can see further and more clearly, but, according to the TMT Corporation, another reason why 30 meters was chosen is because it provides a “sweet spot” for near-infrared studies.

The mirror is not one solid piece of glass. Instead, it’s made of 492 hexagonal segments similar in design to that used in the 10-meter Keck telescopes, also sited at Mauna Kea. Using segments means that building the TMT won’t involve the many problems in constructing a giant mirror – not the least of which would have been transporting such a mirror to Hawaii and then up the mountain, which probably would have meant altering roads. Using segments also means that a broken mirror is an annoyance while a segment is replaced, rather than a catastrophe.

The segments are made of zero-expansion glass or glass ceramic. Each measures about 1.44 meters (56.6 inches) at the corners and they are 45 and 50 mm thick (about 2 inches), with a gap between each segment of only 2.5 mm (0.1 inch).

The TMT primary mirror (Image: Courtesy TMT Observatory Corporation)

Each segment of the primary mirror is held in place by a support system that uses 27 thin flexures that spread the load and prevent the mirror segment distorting under its own weight. This support system can also compensate for temperature changes and shifts in weight as the mirror is tilted.

The secondary mirror that redirects the focused image from the primary mirror gives some idea of the scale of the telescope. The secondary is 3.1 meters (10 feet) in diameter, which is as large as the primary mirror on other telescopes.

There’s also an articulated tertiary mirror in the TMT. It’s job is to steer the light beam from the secondary mirror to any of eight scientific instruments installed in the telescope. The latticework structure of the TMT is supported in two Nasmyth platforms that also house the larger scientific instruments, so the tertiary mirror is necessary to bring these into play.

Only three instruments will be used when it first becomes operational. These include the Wide Field Optical Spectrometer (WFOS), the Infrared Imaging Spectrometer (IRIS) and the Infrared Multi-object Spectrometer (IRMS).

All these components work together as part of the Adaptive Optics (AO) system, which uses the ability of the telescope to reconfigure its elements to compensate for atmospheric turbulence in real time. This turbulence is what makes stars twinkle and it can ruin observations with even a moderate-sized telescope. The AO system, however, can stabilize the image to use the full power of the 30-meter mirror and give the TMT 144 times the light collecting area of the Hubble telescope and ten times better spatial resolution at near-infrared and longer wavelengths.

Covering the light spectrum from 310 nanometers in the ultraviolet to 28 microns in the infrared, the primary mirror has a focal length of f/15 (450 m/1,476 ft) and a focal ratio of f/1.

An artist's interpretation of the TMT (Image: Courtesy TMT Observatory Corporation)

Despite Friday’s ruling, TMT still has hurdles to overcome, such as gaining final approval of construction plans by Hawaii's Department of Land and Natural Resources (DLNR) and negotiating a sublease with the University of Hawaii. If these go according to schedule, TMT Corporation hopes to begin construction in April 2014 and the telescope to be operational by the end of the decade.

The TMT is backed by the University of California, California Institute of Technology, and the Association of Canadian Universities for Research in Astronomy with the National Astronomical Observatory of Japan acting as a Collaborating Institution since 2008.

The video below outlines the TMT project.

Source: Thirty Meter Telescope via Pacific Business News

About the Author
David Szondy David Szondy is a freelance writer based in Monroe, Washington. An award-winning playwright, he has contributed to Charged and iQ magazine and is the author of the website Tales of Future Past.   All articles by David Szondy
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8 Comments

World largest ? And what about the European Extremely Large Telescope ?

witch will be a lot larger.

Etienne Fajitas
17th April, 2013 @ 02:36 am PDT

Now I don't know much of telescopes apart from the optic basics, but I have tried looking through a dumpy level on a construction site with earth-moving equipment moving about and it was shaky at best. Doesn't the volcanic activity in Hawaii create vibrations detrimental to image stability, or is it negligible in this scale?

Leon Van Rensburg
17th April, 2013 @ 02:54 am PDT

@Leon, the higher magnification of a telescope would make any vibrations more detrimental. That said, though, the Keck scopes up there have been used to outstanding ends for many years now; I imagine the seismic activity that Hawaii gets is sporadic enough that the altitude/clear view/lack of light pollution etc. makes it a worthy location.

f8lee
17th April, 2013 @ 09:25 am PDT

Since the mirror is segmented couldn't the segments be adjusted continuously to compensate for atmospheric distortions like some other 'scopes' do?

JAT
17th April, 2013 @ 09:53 am PDT

@Etienne - the EELT is not scheduled to open until mid next decade while this one is hoped to open this decade, so it will be the largest for several years. . .

socalboomer
17th April, 2013 @ 11:02 am PDT

The building will undoubtedly have some real good vibration dampening.

Slowburn
17th April, 2013 @ 01:44 pm PDT

I like the swing away dome panel and the iris petals.

John Hagen-Brenner
18th April, 2013 @ 09:00 am PDT

So, with its 0.001 arcsec capability, we will finally be able to see the lunar lander on the Moon from the surface of the Earth?!

Martin Winlow
23rd April, 2013 @ 02:07 am PDT
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