The TMT Observatory

The core of the TMT Observatory will be a wide-field, alt-az Ritchey-Chretien telescope with a 492 segment, 30 meter diameter primary mirror, a fully active secondary mirror and an articulated tertiary mirror. The optical beam of this telescope will feed a constellation of adaptive optics (AO) systems and science instruments mounted on large Nasmyth platforms surrounding the telescope azimuth structure. These platforms will be large enough to support at least eight different AO/instrument combinations (depending on exact volume and mass parameters) covering a broad range of spatial and spectral resolution. To support and maintain these technical systems, a comprehensive set of support facilities is included in the basic observatory design.

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TMT will couple unprecedented light collection area (almost 10 times more than one of the Keck telescopes) with diffraction-limited spatial resolution that exceeds Keck by a factor of 3. Relative to the Hubble Space Telescope (arguably the most revolutionary astronomical instrument of our generation), TMT will have 144 times the collecting area and more than a factor of 10 better spatial resolution at near-infrared and longer wavelengths.

Aperture Size

The diameter of the primary mirror (M1) is a key design choice – it drives the size and complexity of almost every other observatory system. From a science perspective, we know that the observations limited by atmospheric turbulence improve in sensitivity as diameter squared (D²). Furthermore, AO systems allow us to remove much of the effects of turbulence, achieve diffraction-limited imaging, and gain an improvement of D⁴. Hence, science gain motivates the largest diameter enabled by technology and cost.

Technologically, large M1 diameters are enabled by using the precision-controlled, segmented mirror techniques pioneered with great success by the W. M. Keck Observatory. The TMT M1 design team includes many of the key developers of the Keck system. In principle, arbitrarily large mirrors are possible. However, our analysis of AO technology development over the next ten years with respect to our detailed science goals suggests that D = 30 meters occupies an attractive and achievable scientific "sweet spot" at near-infrared wavelengths.

Obviously, cost also limits M1 diameter – how does financial pressure affect choice of aperture size? Prior to the design of the Keck telescopes, ground-based optical-IR telescope costs scaled roughly as D^2.7, reflecting the nearly volumetric dependence of observatory cost on aperture. The advent of thinner primary mirrors with shorter focal lengths, as in the Keck design, reduced this relationship to an estimated D²; i.e., varying as the area of the aperture. A detailed cost study of key TMT design parameters (aperture, segment count, segment size, segment thickness, and enclosure diameter) for several hundred estimated elements in TMT concluded that TMT costs should scale as approximately D^1.15 relative to the Keck capital investment. Thus, we conclude that the TMT design represents an improved cost scaling and that the proposed 30 meter diameter of TMT represents a good value relative to the extraordinary science reach.

Adaptive Optics

Like all existing ground-based observatories, TMT will be capable of seeing-limited observations, i.e. observations with spatial resolution limited by the natural turbulence in the Earth's atmosphere. Since such observations improve as D², TMT will be able to observe objects nine times fainter than Keck in an equal amount of time.

However, TMT will be the first ground-based astronomy telescope designed with adaptive optics (AO) as an integral system element. AO is a general term that covers systems designed to sense atmospheric turbulence in real-time, correct the optical beam of the telescope to remove its effect, and enable true diffraction-limited imaging on the ground. For many astronomical observations, this is equivalent to observing above the Earth's atmosphere at a fraction of a cost of a space-based observatory. Gains improve from D² to D⁴ – literally opening windows on the nearby and distant Universe inaccessible by any other observatory, on the ground or in space.

Just as the TMT primary mirror builds on the technological and operational heritage of Keck, the TMT adaptive optics design builds on the technological and operational heritage of (among others) the Gemini, Keck, and Very Large Telescope observatories. This is an area of rapid advancement and the TMT project is fortunate to have direct connections to some of the world leaders in this area (e.g. the Center for Adaptive Optics at the University of California, Santa Cruz). TMT plans are based upon an AO development roadmap that takes advantage of AO technological advances as they are being developed and applied in astronomical observations and as they are developed.

The Narrow Field Infrared Adaptive Optics System (NFIRAOS) will be the first adaptive optics system deployed on TMT. The following links will provide more detail in English or in French.

Science Instruments

The TMT design is a response to a set of science-based requirements developed by the TMT Science Advisory Committee (SAC), a committee of scientists representing all the TMT partners and (by extension) the future TMT scientific user community. Central to these requirements are descriptions of a suite of eight instruments conceived to attack the key science problems of the first decade of TMT operations. Starting from these descriptions, detailed conceptual design studies were commissioned from instrument development teams throughout North America. These conceptual designs were reviewed by non-advocate review teams with members from North America and Europe.

Based on these studies and reviews, the SAC has selected three early light instruments: a wide-field, multi-object spectrograph working at optical wavelengths called WFOS; an integral-field unit spectrometer with imaging capability working at near-infrared wavelengths called IRIS; and a multi-slit, near-infrared spectrometer with imaging capability called IRMS. The latter two instruments will be fed by a facility AO system called NFIRAOS to achieve full diffraction-limited sensitivities and spatial resolutions in the near-infrared. These three instruments will be capable of exploring the wide astronomical terrain: from the first stars in the Universe to planets orbiting nearby stars.

The rest of the SAC suite, the first decade instruments, will be developed and deployed on a schedule paced by a combination of technological readiness and available financial resources. Naturally, TMT maintains the flexibility to investigate and deploy different instrument concepts in response to scientific and technological developments over the next decade.

Enclosure

The TMT enclosure has several key functions. By day, it must protect observatory systems, facilitate a broad range of maintenance activities, and keep the telescope temperature near the expected night time temperature. By night, it must shield the telescope from wind buffeting while allowing enough airflow to keep the interior iso-thermal to limit seeing degradation due to air turbulence in the enclosure.

TMT has selected an innovative, structurally efficient calotte enclosure design that, by its spherical shape and circular "shutter" aperture, fulfills key functional requirements with lower mass (and hence lower cost) than possible for previous enclosure designs.

Infrastructure: Summit and Support Facilities

TMT programmatic requirements have been studied to specify the needed conventional facilities and construction sequence at the summit and at a nearby support location. Minimum excavation of the summit to support the required facility footprint and construction allowances has been defined.

The summit facility supports telescope operations, control room functions, staff summit support, mirror handling, stripping, cleaning, storage and recoating, basic engineering and technical activities and other elements of operation that must be carried out close to the telescope.

Support facilities, near the summit, include a construction camp, dormitories and dining facilities for operations staff, and workshops for maintenance and technical activities.