Gemini Multi Conjugate Adaptive Optics (MCAO)

Observatory Sciences consultants have spanned four continents working on the
cutting-edge technology of a new Adaptive Optics system for the Gemini Observatory.

Adaptive Optics (AO) is a technology to improve the performance of telescopes by reducing the effects of rapidly changing distortions due to atmospheric turbulence, known to astronomers as 'seeing'. Adaptive Optics works by measuring the distortion and rapidly compensating for it using deformable mirrors. However, a bright reference star is needed close to the target being studied which results in only a small proportion of the sky being available for study using the original AO systems. Further developments include the use of Laser Guide Stars to produce an artificial reference star high in the atmosphere. However, the effective field of view remains small.

It is essential that Adaptive Optics be usable over a much larger part of the sky and with wider fields of view when the next generation of giant telescopes (with mirror diameters larger than 20 metres) come into operation. Multi Conjugate Adaptive Optics (MCAO) solves most of these problems. By using several guide stars and several deformable mirrors, a uniform image compensation can be achieved on a larger field using natural or laser guide stars. By probing and correcting a larger volume of turbulent atmosphere, MCAO increases the compensation performance on current 8m telescopes and opens up new possibilities for adaptive optics on future giant telescopes.

The Gemini Observatory is planning to implement a MCAO system as a facility instrument for the Gemini South 8m telescope in Chile. The system will include five laser guide stars, three natural guide stars, and three deformable mirrors configured to analyse atmospheric turbulence at different altitudes to achieve atmospheric compensation over a 1 arc minute square field of view.

Control software

OSL produced the control software for four of the subsystems that make up the Gemini MCAO:

- The Beam Transfer Optics (BTO) system, which launches the laser beam.

- The Beam Transfer Optics Diagnostics Sensor System, which provides BTO mirror corrections.

- The MCAO Slow Focus Wavefront Sensor (SFO) which computes the slowly varying focus corrections from a natural guide star that the laser guide star (LGS) cannot provide.

- The Adaptive Optics Module (AOM), which contains all the optics and sensors to compensate the input beam and relay it to the instrument.

This involves OSL staff writing the software in Cambridge, UK, testing the first Laser Guide Star at Gemini North in Hawaii, and installing it at Gemini South in Chile.

In Autumn 2005, OSL was contracted by the Australian National University to provide on-site consultancy effort to complete and commission the control software for the Gemini South Adaptive Optics Imager. This 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.

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