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GeMS First Light: A New Generation of Adaptive Optics Begins

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Figure 1. The engineering first light image obtained with GeMS and the Gemini South Adaptive Optics Imager GSAOI on April 19, 2011. The field of view is 85 x 85 arcseconds and the wavelength is 2.12 microns. Strehl ratio and full-width at half-maximum values for all stars are shown in the insets. This image was obtained after only 30 minutes of focusing and optimization – it is by no means representative of what is expected from the fully-commissioned system. However, it is very encouraging and already illustrates the main advantage of MCAO, which is a relatively uniform compensation across a large field of view. Note that the poorer image quality on the left edge is expected, as these stars are outside the constellation defined by the three bright stars in the right half used to control Tip-Tilt.

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In early April 2011, after more than a decade of effort, the Gemini Multi-Conjugate Adaptive Optics System (GeMS) saw starlight for the first time. With this milestone GeMS kicked off a new era in adaptive optics (AO) technologies both for Gemini and for future generations of even larger telescopes that will require advanced AO to make them scientifically viable. These first GeMS photons, captured on April 19, 2011 (see Figure 1), kept hopes high for equally successful progress when commissioning resumes in late 2011 and then for system verification in early 2012.

Adaptive Optics is a well-known technology that compensates for image distortions induced by atmospheric turbulence. The vast majority of large telescopes in the world are now equipped with AO systems of various kinds, and many use lasers to create artificial stars (laser guide stars, LGS) in order to probe and correct for atmospheric turbulence over a larger portion of the sky (e.g. Altair at Gemini North). Multi-Conjugate Adaptive Optics (MCAO) is a relatively novel concept, in which the distortions are compensated by using not one, but a series of deformable mirrors and multiple guide stars. In effect, this provides compensation in three dimensions versus the two dimensions for classical (existing) AO systems. The impact on data is that there is a ten-fold increase in the size of the corrected field of view and significantly more uniform corrections across the entire field than classical AO. GeMS is the first instrument to use MCAO with multiple laser guide stars – thus explaining the long development cycle which began in 2001.

In March 2010 Gemini received the 50-watt sodium laser built by Lockheed Martin Coherent Technology. Following this, Canopus, the GeMS optical bench, made its much-anticipated trek from the Gemini South Base Facility integration laboratory to the Cerro Pachón summit in November 2010. The installation on the telescope in January 2011 marked the beginning of a five-month commissioning marathon for the Gemini MCAO team. The first two commissioning periods – one week in January and one in February – were dedicated to the laser and associated subsystems, beam transfer optics, laser launch telescope and various laser safety systems. The laser generally performed well, delivering between 45 and 60 watts of yellow (sodium) light (see the June 2011 issue of GeminiFocus, pages 23-28). After initial concerns about the laser guide star spot size we regularly obtained spot sizes of 1.3 arcseconds as viewed by the telescope’s acquisition camera system.

The commissioning of Canopus started in March, with two subsequent runs in April and May, for a total of nine clear nights. To date, the commissioning has concentrated on functionality since this very complex instrument includes multiple subsystems linked together by closed loops and offloads that the MCAO team had to characterize, debug and optimize. Currently, almost all subsystems, loops and offloads are working, which include: the main LGS loop, the natural guide star (NGS) loops, all telescope offloads, the LGS and NGS acquisition sequences, and many others. The technical first light image shown in Figure 1 was obtained on April 19 with plans to continue commissioning through May. However, bad weather and various technical issues prevented making much more progress in areas such as performance optimization and characterization as well as plans to acquire additional astronomical images which would demonstrate the system’s capabilities more dramatically.

In early June 2011, GeMS entered a five-month rework period. Significant work on the laser to insure better beam quality, stability and reliability is planned. Stability issues in the Beam Transfer Optics will also be addressed. Finally, various Canopus subsystems will be upgraded, with a particular effort to improve the optical throughput of the natural guide star wavefront sensors. Commissioning should resume at the end of 2011, with a focus on performance and integration with the observatory high-level software. System Verification (SV) is expected to occur in the first half of 2012.

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