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Thirty Meter Telescope Project Issues Request for Proposals for Facility Adaptive Optics System

Pasadena, California – February 18, 2005:   The Thirty Meter Telescope project continues to build momentum as it issues Requests for Proposals for its first generation instruments, including the NFIRAOS facility adaptive optics system. The announcement solicits proposals for a Conceptual Design Study of
  NFIRAOS (pronounced “nefarious”), a multi-conjugate laser guide star AO system that will provide the TMT's facility adaptive optics capability.

NFIRAOS (the Narrow Field Infra-Red Adaptive Optics System) will be used to feed three TMT instruments as they become available, but its first role will be to provide AO correction for IRIS, an IR diffraction-limited imaging spectrometer operating across the 1.0µm – 2.5µm spectral range. IRIS will include a 2 arcsec integral field unit and a 10”×10” imager.

Two more instruments will come later, NIRES, a Near Infrared Echelle Spectrograph for 1µm – 5µm high resolution spectroscopy, and WIRC, a Wide Field (30”) Infrared Camera also operating over the 1µm – 5µm near-infrared spectral region.

NFIRAOS will be a dual-conjugate AO system. A 30 arcsec corrected field of view is achieved with two deformable mirrors conjugated to 0km and 12km
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altitude, respectively. The mirrors use piezo-stack actuators of conventional design to minimise technical risk. Both deformable mirrors have 60 active actuators across the telescope aperture. The laser guide star system uses up to 9 sodium laser beacons of 25 W power, each with an associated wavefront sensor, along with one or more natural guide star wavefront sensors for tip-tilt sensing.
 
The Thirty Meter Telescope (TMT) will be the first of the next-generation of extremely large optical/infrared ground-based observatories. When completed, it will be the largest telescope in the world. The TMT's design builds on the experience gained with the two Keck telescopes. It has a segmented primary mirror made up of 738 hexagonal segments, each 1.2 m across. Its location will be decided once site testing is complete. Telescope construction is expected to start in 2008, with science operations beginning in 2014.

The Thirty Meter Telescope project consortium includes the two partners in the Keck Observatory, the University of California and the California Institute of Technology, together with the U.S. Association of Universities for Research in Astronomy (AURA) and a Canadian national consortium, the Association of Canadian Universities for Research in Astronomy (ACURA).

While NFIRAOS will serve as the facility adaptive optics system for the Thirty Meter Telescope, four further first-generation TMT adaptive
TMT An artist's impression of the Thirty Meter Telescope, with its Laser Guide Star.

Image:   Todd Mason, Mason Productions
optics systems are planned, to meet the requirements of other key instruments:
 
•   GLAO, the Ground Layer Adaptive Optics system, will correct only low altitude turbulence, thus providing a lower degree of correction, but over a very wide field of view. It will feed the WFOS instrument, a wide field (8–20 arcmin) optical (0.31–1 µm) spectrograph with 150<R<6000 spectral resolution.  
GLAO will use either the telescope's adaptive secondary mirror or an 855-actuator deformable mirror for wavefront correction, together with 5–9 laser guide star wavefront sensors, averaged down to form a single-conjugate AO system.
 
 
 
TMT
The Thirty Meter Telescope, and a comparison with the Hale and Keck primary mirrors.

Drawing:   Doug Cummings, Caltech
Laser Guide Star Adaptive Optics, Ageorges
•   MOAO, the Multi-Object Adaptive Optics system, will feed the IRMOS multi-object spectrograph. IRMOS will perform near-diffraction limited spectroscopy in a wavelength range of 0.8 µm to 2.5 µm (using up to 20 integral field units with R~2000–10000) over a 5 arcmin diameter field.  
Each of the 10–20 individual fields of view will be corrected by its own MEMS deformable mirror, and each of these MEMS mirrors will have up to 100×100 (10,000) actuators. The wavefront sensing will use a 9 laser guide star asterism.
 
•   MIRAO, the Mid-IR AO system, will feed the MIRES instrument, an IR spectrograph whose function is to perform diffraction limited spectroscopy in the range of 5 µm to 28 µm with a spectral resolution between 5000<R<100000.  
Like GLAO, it will use either the adaptive secondary or an 855-actuator DM, and 1 or 3 LGS wavefront sensors.
 
Extrasolar Planets, Beaulieu
•   ExAO, the Extreme Adaptive Optics system, will feed the PFI planet finder instrument, which will perform very high-contrast imaging and low spectral resolution IFU (integral field unit) spectroscopy in the 1 to 2.5 µm range for direct planet detection and characterization.  
ExAO will be a natural guide star AO system, since a bright central star will always be available for wavefront sensing. It will use the TMT's adaptive secondary as a low order ("woofer")
correcting element, and a MEMS mirror for high order "tweeter" correction. The order of correction will be exceptionally high, with a Mach-Zehnder phase shifting interferometer being used for the high order wavefront sensing, and a MEMS mirror with up to 200,000 elements for the wavefront correction.
 
NFIRAOS itself will also be upgraded at a later date: The number of actuators on the ground-conjugated DM will be raised four-fold, from 60 actuators across the aperture to 120 across. The number of subapertures on the LGS wavefront sensors, and the laser power, will also be quadrupled. An adaptive secondary mirror will be fitted to the telescope, which NFIRAOS will use as a low order "woofer" corrector, providing more mirror stroke.


 

© 2006
 
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