Extremely large telescopes ELTs

The largest optical telescopes in the world are currently the 10 m Keck telescopes on Mauna Kea, Hawaii. Astronomers always want telescopes with ever larger apertures in order to image fainter and fainter objects, but the size of new telescope plans currently being considered is astonishing in their ambition! The next generations of telescopes, called "extremely large telescopes", are likely to have apertures of at least 30 m. Apart from their enormous light-gathering power, with adaptive optics such telescopes would have fine enough angular resolution to greatly improve observations of solar system objects such as the giant planets and would even be able to directly image extrasolar planets and, by measuring their visible and near-IR spectra, place constraints on their atmospheric composition.

Such enormous telescopes are not the stuff of science fiction and detailed plans and proposals are already being considered. Almost all use the idea of segmented mirrors, along the design of the Keck telescopes, although it may now be technically feasible to make a monolithic mirror as large as 50 m in diameter! The multi-mirror approach will probably use a spherical primary since all the individual elements would then be ground to the same, simple shape. The disadvantage is, of course, spherical aberration, which would need to be corrected for by additional optics, slightly reducing the overall throughput through reflective losses. Making a parabolic mirror with individual elements is technically possible, but much more difficult and expensive since the shape of the mirrors will differ depending on their final position in the assembled primary.

There are currently a number of ELT designs under serious consideration.

European Extremely Large Telescope (E-ELT)

The European Southern Observatory is currently considering plans for a 30 m to 50 m diameter ELT. An earlier study into the so-called "Overwhelmingly Large Telescope (OWL)", which was a concept to construct a ground-based 100 m class optical and near-IR adaptive optics telescope, has for the time being been shelved due to the extremely high projected costs.

The current E-ELT design is for an alt-azimuthal telescope with a segmented, aspherical 42 m primary mirror and then one of two secondary mirror systems: (1) a Gregorian system with a 4.8 m concave secondary and also perhaps a 5 m flat tertiary mirror to bring the light to the Nasmyth focus, or (2) a new optical design with an active convex 6 m secondary followed by a concave, aspheric tertiary and two flat mirrors to relay the beam to the Nasmyth focus. The use of a segmented mirror with individual mirrors no larger than 1.45 m reduces the risks and projected costs, and it is clear that such a system will require active optics to maintain optical quality, which has already been successfully demonstrated by the ESO New Technology Telescope (NTT) and VLT. Compensation for atmospheric turbulence will be taken care of by multi-conjugate adaptive optics modules located between the telescope focus and the science instruments. The telescope structure is designed to be all-steel and would weigh something in the region of 5,500 tonnes. The telescope would probably operate in the open air, since an enclosure with a 42 m wide slit would not provide much protection from the wind anyway, and hence in operation the enormous enclosure would probably roll right away. Modeling the possible vibrations induced by surface winds, which would severely affect optical quality, is one of the main activities of the current study.

Thirty Meter Telescope (TMT)

In a project formerly known as the California Extremely Large Telescope (CELT) (Irion, 2002) the TMT Consortium—made up of the California Institute of Technology, the University of California, and the Association of Canadian Universities for

Research in Astronomy (ACURA)—are designing a 30 m class telescope, which they hope to begin constructing in 2009 and achieve "first light" by 2016. The current design is for an alt-azimuth mounted Ritchey-Chrétien telescope with a 492-segment, 30 m primary mirror, an active secondary mirror, and an articulated tertiary mirror and will use adaptive optics. To maximize performance, such a telescope needs to be built in a dry region at high altitude and a number of possible sites are currently being considered.

Giant Magellan Telescope (GMT)

The Giant Magellan Telescope (GMT) Organization consists of the Carnegie Institution of Washington, Harvard University, the Smithsonian Astrophysical Laboratory, Texas A&M University, the University of Texas at Austin, the Australian National University, the Univerisity of Arizona, and Astronomy Australia Ltd. The consortium plan to build an ELT consisting of seven 8.4 m mirrors at a site in Chile, and hope to complete in around 2017.

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