Michael Sheehan

Gemini multiconjugate adaptive optics system review - I. Design, trade-offs and integration

The Gemini multiconjugate adaptive optics system (GeMS) at the Gemini South telescope in Cerro Pachon is the first sodium-based multilaser guide star (LGS) adaptive optics system. It uses five LGSs and two deformable mirrors to measure and compensate for atmospheric distortions. The GeMS project started in 1999, and saw first light in 2011. It is now in regular operation, producing images close to the diffraction limit in the near-infrared, with uniform quality over a field of view of two square arcminutes. This paper is the first one in a two-paper review of GeMS. It describes the system, explains why and how it was built, discusses the design choices and trade-offs, and presents the main issues encountered during the course of the project. Finally, we briefly present the results of the system first light.

Giant Magellan Telescope: overview

The Giant Magellan Telescope (GMT) is a 25-meter optical/infrared extremely large telescope that is being built by an international consortium of universities and research institutions. It will be located at the Las Campanas Observatory, Chile. The GMT primary mirror consists of seven 8.4-m borosilicate honeycomb mirror segments made at the Steward Observatory Mirror Lab (SOML). Six identical off-axis segments and one on-axis segment are arranged on a single nearly-paraboloidal parent surface having an overall focal ratio of f/0.7. The fabrication, testing and verification procedures required to produce the closely-matched off-axis mirror segments were developed during the production of the first mirror. Production of the second and third off-axis segments is underway. GMT incorporates a seven-segment Gregorian adaptive secondary to implement three modes of adaptive-optics operation: natural-guide star AO, laser-tomography AO, and ground-layer AO. A wide-field corrector/ADC is available for use in seeing-limited mode over a 20-arcmin diameter field of view. Up to seven instruments can be mounted simultaneously on the telescope in a large Gregorian Instrument Rotator. Conceptual design studies were completed for six AO and seeing-limited instruments, plus a multi-object fiber feed, and a roadmap for phased deployment of the GMT instrument suite is being developed. The partner institutions have made firm commitments for approximately 45% of the funds required to build the telescope. Project Office efforts are currently focused on advancing the telescope and enclosure design in preparation for subsystem- and system-level preliminary design reviews which are scheduled to be completed in the first half of 2013.

GeMS: first on-sky results

GeMS, the Gemini Laser Guide Star Multi-Conjugate Adaptive Optics facility system, has seen first light in December 2011, and has already produced images with H band Strehl ratio in excess of 35% over fields of view of 85x85 arcsec, fulfilling the MCAO promise. In this paper, we report on these early results, analyze trends in performance, and concentrate on key or novel aspects of the system, like centroid gain estimation, on-sky non common path aberration estimation. We also present the first astrometric analysis, showing very encouraging results.

Overview and status of the Giant Magellan Telescope project

The Giant Magellan Telescope (GMT) is a 25.4-m diameter, optical/infrared telescope that is being built by an international consortium of universities and research institutions as one of the next generation of Extremely Large Telescopes. The primary mirror of GMT consists of seven 8.4 m borosilicate honeycomb mirror segments that are optically conjugate to seven corresponding segments in the Gregorian secondary mirror. Fabrication is complete for one primary mirror segment and is underway for the next two. The final focal ratio of the telescope is f/8.2, so that the focal plane has an image scale of 1.02 arcsec/mm. GMT will be commissioned using a fast-steering secondary mirror assembly comprised of conventional, rigid segments to provide seeing-limited observations. A secondary mirror with fully adaptive segments will be used in standard operation to additionally enable ground-layer and diffraction-limited adaptive optics. In the seeing limited mode, GMT will provide a 10 arcmin field of view without field correction. A 20 arcmin field of view will be obtained using a wide-field corrector and atmospheric dispersion compensator. The project has recently completed a series of sub-system and system-level preliminary design reviews and is currently preparing to move into the construction phase. This paper summarizes the technical development of the GMT sub-systems and the current status of the GMT project.

GeMS: Gemini Mcao System: current status and commissioning plans

The Gemini Multi-Conjugate Adaptive Optics project was launched in April 1999 to become the Gemini South AO facility in Chile. The system includes 5 laser guide stars, 3 natural guide stars and 3 deformable mirrors optically conjugated at 0, 4.5 and 9km to achieve near-uniform atmospheric compensation over a 1 arc minute square field of view. Sub-contracted systems with vendors were started as early as October 2001 and were all delivered by July 2007, but for the 50W laser (due around September 2008). The in-house development began in January 2006, and is expected to be completed by the end of 2008 to continue with integration and testing (I&T) on the telescope. The on-sky commissioning phase is scheduled to start during the first half of 2009. In this general overview, we will first describe the status of each subsystem with their major requirements, risk areas and achieved performance. Next we will present our plan to complete the project by reviewing the remaining steps through I&T and commissioning on the telescope, both during day-time and at night-time. Finally, we will summarize some management activities like schedules, resources and conclude with some lessons learned.

High-power solid-state sodium guidestar laser for the Gemini North Observatory

We report on the first successful installation of a commercial solid-state sodium guidestar laser system (GLS). The GLS developed at LMCT was delivered to Gemini North Observatory in February of 2005. The laser is a single beacon system that implements a novel laser architecture and represents a critical step towards addressing the need of the astronomy and military adaptive optics (AO) communities for a robust turn-key commercial GLS. The laser was installed on the center section of the 8 m Gemini North telescope, with the output beam relayed to a laser launch telescope located behind the 1 m diameter secondary mirror. The laser went through a three week performance evaluation between November and December 2005 wherein it consistently generated 12 W average power with measured M2 < 1.1 while locked to the D2 line at +/- 100 MHz. The system was required to perform during a 12-hour test period during three runs of 4-6 consecutive nights each. The laser architecture is based on continuous wave (CW) mode-locked solid-state lasers. The mode-locked format enables more efficient SFG conversion, and dispenses with complex resonant intensity enhancement systems and injection-locking electronics. The linearly-polarized, near-diffraction-limited, modelocked 1319 nm and 1064 nm pulses are generated in separate dual-head diode-pumped resonators. The two IR pulses are input into a single-stage, 30 mm PPSLT sum-frequency generation (SFG) crystal provided by Physical Science, Inc. Visible (589 nm) power of >16 W have been generated, representing a conversion efficiency of 40%.

Wind-induced structural response of a large telescope

In May of 2000, the construction progress of the Gemini South 8m telescope at Cerro Pachon in Chile was such that the telescope and dome were installed and able to move, but the primary mirror had not been installed. This provided a unique opportunity to make extensive tests of the structure in its nearly-completed state, including a modal impact test and simultaneous measurements of wind pressure and structural response. The testing was even more comprehensive because the Gemini dome design allows for a wide range of wind flow configurations, from nearly enclosed to almost fully exposed. In these tests, the operating response of 24 surface pressures on the primary mirror cell, 5 wind velocity channels (each with direction vector information), and more than 70 channels of accelerometers on the telescope structure were measured. The data were taken in a variety of wind loading configurations. While previous analysis efforts have focused on the wind velocity and pressure measurement, this paper investigates the dynamic behavior of the telescope structure itself. Specifically, the discussion includes the participation of the modes measured in the modal impact test as a function of wind loading configuration. Data that indicate the most important frequency ranges in the operating response of the telescope are also presented. Finally, the importance of the response of the enclosure on the structural vibration of the telescope structure is discussed.

Vibration mitigation for wind-induced jitter for the Giant Magellan Telescope

The Giant Magellan Telescope (GMT) is a planned large terrestrial telescope with a segmented primary mirror with a 24.5 meter overall diameter. Like most terrestrial telescopes, the GMT resides within an enclosure designed to protect the telescope from the elements and to reduce the effects of wind on the optical performance of the telescope. Wind impingement on the telescope causes static deformation and vibration in the telescope structure that affects the alignment and image jitter performance of the telescope. Actively controlled primary mirror segments and a secondary mirror can correct for the static and low frequency portions of the wind effects, but typically the actuators do not have the bandwidth to address higher frequency components of the wind environment. Preliminary analyses on the GMT indicate that the image jitter associated with wind effects meets budgeted allowances but without much margin. Preliminary models show that the bulk of the residual jitter arises from excitation of a small number of modes in the 9 to 12 Hz range. Therefore, as a risk mitigation effort to increase the margin on the wind induced jitter, passive and active vibration mitigation approaches have been examined for the GMT, which will be the focus of this paper. Using a finite element model of the GMT along with wind loading load cases, several passive and active vibration mitigation approaches were analyzed. These approaches include passive approaches such as tuned mass dampers targeting the worst offending modes, and constrained layer damping targeting all of the modes within the troublesome frequency range. Active approaches evaluated include two active damping approaches, one using several reaction mass actuators and the other using active strut type actuators. The results of the study show that although all approaches are successful in reducing the jitter, the active damping approach using reaction mass actuators offers the lightest weight, least implementation impact, and most adaptability of any of the approaches.

High chi polymer development for DSA applications using RAFT technology

Directed self-assembly (DSA) of block copolymers is proving to be an interesting and innovative method to make three-dimensional periodic, uniform patterns useful in a variety of microelectronics applications. Attributes critical to acceptable DSA performance of block copolymers include molecular weight uniformity, final purity, and reproducibility in all the steps involved in producing the polymers. Reversible Addition Fragmentation Chain Transfer (RAFT) polymerization technology enables the production of such materials provided that careful process monitoring and compositional homogeneity measurement systems are employed. It is uniquely suited to construction of multiblocks with components of widely divergent surface energies and functionality. We describe a high chi diblock system comprising partially fluorinated methacrylates and substituted styrenics. While special new polymer separation strategies involving controlled polymer particle assembly in liquid media are required for some monomer systems and molecular weight regimes, we have been able to demonstrate high yield and compositionally homogeneous diblocks of lamellar and cylindrical morphology with polydispersities < 1.1. During purification processes, these diblock materials undergo assembly processes in liquid media, and with appropriate controls, this allows for removal of soluble homopolymer contaminants. SAXS analyses of solid polymer samples provide estimates of lamellar d-spacing, and a good correlation with molecular weight is shown. This system will be described.

Progress on the structural and mechanical design of the Giant Magellan Telescope

The Giant Magellan Telescope (GMT), one of several next generation Extremely Large Telescopes (ELTs), is a 25.4 meter diameter altitude over azimuth design set to be built at the summit of Cerro Campanas at the Las Campanas Observatory in Chile. The primary mirror consists of 7 individual 8.4 meter diameter segments resulting in an equivalent collecting area of a 21.5 meter diameter single mirror. The telescope structure, optics and instrumentation has a rotating mass of approximately 1250 metric tons and stands approximately 40 meters tall. This paper reports the results of our ongoing preliminary design and development of the GMT structure and its major mechanical and opto-mechanical components. A major recent redesign of the Gregorian Instrument Rotator (GIR) resulted in significant changes to the telescope structure and several mechanisms. Design trade studies of various aspects of the main structure, hydrostatic bearing system, main axes drives, M2 positioner, M3 subsystem and the corrector-ADC subsystem have refined the preliminary design in these areas.