The Australian National University

Gemini South Adaptive Optics Imager (GSAOI)

Instrument Description:

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The Gemini South Adaptive Optics Imager (GSAOI) is a near-infrared adaptive optics camera that will be used with the Multi-Conjugate Adaptive Optics (MCAO) system on the Gemini South 8-m telescope.

GSAOI is being designed and constructed by the Research School of Astronomy and Astrophysics (RSAA) of The Australian National University (ANU). GSAOI is the second instrument that RSAA has built for the Gemini Observatory. GSAOI is one component of RSAA's involvement in the Major National Research Facilities Program to increase Australian Gemini access.

The Gemini Observatory operates two 8-m diameter telescopes; Gemini North on Mauna Kea in Hawaii and Gemini South on Cerro Pachon in Chile. The Gemini telescopes are owned by an international consortium involving the USA, UK, Canada, Australia, Argentina, Brazil, and Chile. Australia is a 5% partner in this consortium. Australian membership is funded by the Australian Research Council and the Commonwealth Department of Education, Science and Training through the Major National Research Facilities component of the Backing Australia's Ability program.

Gemini's major instrumentation goal is to develop its Multi-Conjugate Adaptive Optics (MCAO) system for scientific use on Gemini South. Turbulence in the Earth's atmosphere blurs images of astronomical objects seen through ground-based telescopes. By using sophisticated optical systems, it is possible to rapidly measure and correct this blurring so that sharp images are obtained that are limited only by fundamental diffraction in the telescope. Existing adaptive optics systems use a single natural guide star or artifical laser guide star to measure this image blur. These systems provide full correction only within about 20 seconds of arc of the guide star. The MCAO system will use three natural guide stars and five artifical laser guide stars to correct images over a region of sky approximately 80 seconds of arc in diameter.

The Gemini South Adaptive Optics Imager is a diffraction-limited camera that will be used with the MCAO system to record extremely sharp images with resolutions similar to those obtained with the Hubble Space Telescope over similar-sized fields on the sky and at much lower cost.

The MCAO system achieves its best correction at near-infrared wavelength between 1 and 2.5 microns, redder than the eye can see. Consequently, GSAOI will operate in this near-infrared wavelength region. It will use a mosaic of four HAWAII-2RG near-infrared detectors developed by the Rockwell Science Center for the Next Generation Space Telescope. These detectors will record 4096×4096 pixels of information in a single exposure. GSAOI will have superb optics in order not to disturb the exceptional images produced by MCAO. The image scale will be 0.02 seconds of arc per pixel so that GSAOI adequately samples these diffraction-limited images. The full detector mosaic will record a square field 85 seconds of arc on a side.

GSAOI will be a facility Gemini instrument. As such, its operation will be fully integrated with the Gemini telescope control system, and it must meet the weight and space requirements of facility-class instruments. Three such instruments can be mounted on the back of each telescope at any one time. Each instrument weighs 2 tons.

The Research School of Astronomy and Astrophysics proposed in September 2001 to perform one of two Conceptual Design Studies for GSAOI. The Gemini Observatory selected RSAA and the USA National Optical Astronomy Observatories (NOAO) in November 2001 to perform these design studies. Two Conceptual Design Reviews were held in August 2002 in Hawaii. RSAA was selected in November 2002 to perform the detailed design and construction of the instrument.

GSAOI will be used to commission the MCAO system on Gemini South in 2005. The development of the instrument will be fast tracked in order to meet this demanding deadline.

The Research School of Astronomy and Astrophysics is well placed to undertake this task in that they are nearing completion of the construction of a similar near-infrared adaptive optics instrument for Gemini North. This instrument is the Gemini Near-infrared Integral Field Spectrograph (NIFS). NIFS is the first instrument that Australia has built for the Gemini telescopes. RSAA won the contract to build this instrument in 2000. NIFS will be commissioned on Gemini North in July 2003.

Much of GSAOI will be duplicated from NIFS in order to speed its development.