Adaptive optics (AO) is a well-known technology that compensates for image distortions induced by atmospheric turbulence. Most of the world's large telescopes are now equipped with AO systems of various kinds, and many use lasers to create artificial stars (laser guide stars, LGSs) to probe and correct for atmospheric turbulence over a larger portion of the sky. Multi-conjugate adaptive optics (MCAO) is a relatively novel concept1, 2 that compensates for distortions using a series of deformable mirrors and multiple guide stars. The technique samples the turbulence structure in the atmosphere at several levels and then, similarly to medical tomography, reconstructs a 3D snapshot of the effect of the atmosphere on starlight. This snapshot is then used to shape a series of deformable mirrors to cancel out the distortion. All of this happens about a thousand times a second. In effect, MCAO provides compensation in three dimensions as opposed to two for classical (existing) AO systems. The impact on data is a 10-fold increase in the size of the corrected field of view and significantly more uniform corrections across the entire field. MCAO opens new windows in the sky for objects previously unobservable using conventional approaches.
The Gemini South observatory, located in Chile, is currently in the process of commissioning the first instrument to use MCAO with multiple laser guide stars: the Gemini MCAO System, also called GeMS.3, 4 Figure 1 shows the laser propagating above the Gemini telescope in Cerro Pachón, Chile. Figure 2 shows a closer view of the five-laser guide star ‘constellation’ used to sample atmospheric turbulence. This artificial constellation is produced by a 50W laser split into 5×10W beacons,5 propagating upward toward the atmospheric sodium layer about 90km above the Earth's surface. The interaction of the laser light with the sodium layer creates a distinctive five-point grouping (see Figure 2) that resembles the pattern on a single die or domino. The yellow-orange beam visible from lower right to upper left is caused by scattering of the laser light by the earth's lower atmosphere. These five LGSs are seen by five 16×16 subapertures—Shack-Hartmann wavefront sensors (WFSs)—that are used to compute the MCAO high-order correction. The latter is applied at 800Hz by three deformable mirrors totaling 917 actuators and optically conjugated to 0, 4.5, and 9km, respectively. This entire process is the heart of MCAO correction. In addition, up to three either visible or near-IR (NIR) natural guide stars (NGSs) provide measurements for correcting image stabilization, also referred to as tip-tilt and anisoplanatic modes.
Figure 1. The Gemini South telescope on the night of 21–22 January 2011 during the first propagation of the Gemini Multi-Conjugate Adaptive Optics System (GeMS) laser guide star system on the sky. (Photo credit: Gemini Observatory/Association of Universities for Research in Astronomy Inc., AURA)
Figure 2. Laser constellation and Rayleigh diffusion. (Photo credit: M. Boccas.)
On-sky commissioning started in January 2011 at a frequency of around one week per month around the full moon. The first months of work were dedicated to GeMS functionality: this very complex instrument includes multiple subsystems linked together by closed loops and offloads that the team had to characterize, debug, and optimize. First results were obtained as soon as April 2011, with the first images demonstrating the system capabilities. In early June 2011, GeMS entered a planned five-month rework period. Since wintertime in Chile is the least favorable for AO observations, we took advantage of this perfect opportunity to fix, repair, and upgrade many systems based on our on-sky experiences.
Commissioning resumed in November, and since December 2011, GeMS has been producing the first science images corrected by MCAO. Figure 3 shows one of the first of these acquired by the Gemini South Adaptive Optics Imager (GSAOI), a large-field-of-view NIR camera. It targets a portion of the globular cluster NGC288. The field of view on this image is 87×87 arcseconds, which is about 10 times larger that any other current AO system. These images clearly show the gain brought by MCAO. Performance at that point was still limited by static residual aberrations, and the Strehl ratio (SR) measured in this image is around 15% (H-band), but with a variation of only a few percent across the whole field. Since then, performance has improved. An SR of 35% in the H-band (full width at half-maximum of 50 milli-arcseconds) is typically achievable and never before obtained with LGS systems. Moreover, GeMS provided it over a field of view 10 times larger than that of other AO systems around the world.
Figure 3. Central part of NGC288, GeMS/Gemini South Adaptive Optics Imager (GSAOI) first light image, obtained at 1.65 microns (H-band) on 16 December 2011. The field of view is 87×87 arsceconds. Exposure time is 13 minutes. The average full width at half-maximum (FWHM) is slightly below 0.080 arcseconds, with a variation of 0.002 arcseconds across the entire image. Insets on the right show a detail of the image (up), and a comparison of the same region with classical AO (middle: this assumes using the star at the upper right corner as the guide star) and seeing-limited observation (bottom).The pixel size in the latter was chosen to optimize the signal-to-noise ratio while not degrading the intrinsic angular resolution of the image. (Photo credit: Gemini Observatory/AURA.)
In 2012, the system will finish commissioning with a focus on stability, optimizing performance, and integrating into operations. It will gradually be opened to the Gemini astronomical community over the course of the year, providing remarkably sharp images for the study of a wide range of topics from the birth and evolution of stars to the dynamics of distant galaxies. GeMS will ‘feed’ a variety of instruments that work in the NIR part of the spectrum and produce images and spectra of objects previously unobservable at this level of clarity. Eventually, we expect the Gemini system to set the stage for the next generation of large ground-based telescopes, which will have mirrors 30m in diameter or more. These telescopes will require the latest AO technologies to produce images of sufficient resolution given the wide column of air they will observe through.
This work was supported by the international Gemini partnership funding agencies, which include the US National Science Foundation (NSF), the UK Science and Technology Facilities Council, the Canadian National Research Council, the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica, the Australian Research Council, the Argentinean Consejo Nacional de Investigaciones Cientificas y Técnicas, and the Brazilian Conselho Nacional de Desenvolvimento Cientifico e Tecnológico CNPq. The Gemini observatory is managed by the Association of Universities for Research in Astronomy Inc. under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.
Benoit Neichel, Francois Rigaut
La Serena, Chile
Benoit Neichel is an adaptive optics scientist. He is the instrument scientist of the Gemini MCAO system.
Francois Rigaut is the adaptive optics senior scientist at Gemini. He is the project scientist of the Gemini MCAO system.
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