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The ULTIMATE Ground Layer Adaptive Optics (GLAO) system is part of ULTIMATE-Subaru, the next generation facility instrumentation project at the Subaru telescope in Hawaii. GLAO is led by the Subaru Telescope in collaboration with Tohoku University, Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), and the Australian National University (ANU).
ANU is currently designing the Laser Guide Star Facility and the Wavefront Adaptor Flange with four Laser Guide Star wavefront sensors, and four Natural Guide Star wavefront sensors. The GLAO Wavefront Adaptor Flange will provide Adaptive Optics capability to the wide-field imager (WFI) instrument to be installed at the Subaru Cassegrain focus in 2027.
Four Laser Guide Star wavefront sensors mounted over a fabricated steel structure enable the acquisition of LGS asterisms of up to 20 arcmin in diameter. Each WFS has been designed to also account for the telescope optical aberrations and the non-telecentricity. The NGS instance consists of four Natural Guide Star wavefront sensors for tip-tilt and focus measurement.
In this paper, we present an overview of the GLAO Wavefront Adaptor Flange including the preliminary design for the opto-mechanical assembly of both LGS and NGS instances, and the mechanisms control system that enables fine acquisition of the guide stars over the wide patrol field of the GLAO system.
The Ground Layer Adaptive Optics (GLAO) system for ULTIMATE, the next generation instrumentation project for the Subaru telescope, will generate and use four laser guide stars on sky in side-launch configuration. The design of the GLAO is led and coordinated by the Subaru telescope in collaboration with Tohoku University, Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), and the Australian National University (ANU). ANU is responsible for the wavefront sensor subsystem and the Laser Guide Star Facility.
The GLAO Laser Guide Star Facility (LGSF) includes two Sodium guidestar lasers to be split in a total of four, generating an asterism of four artificial stars on the Hawaiian skies. Divided into three main subsystems (beam transfer optics, beam diagnostics, and beam projection), the GLAO LGSF accounts for the conditioning, splitting, and steering of the laser beams as well as for their launching configuration over a patrol field of 20 arcmin on sky.
This paper presents the preliminary design of the GLAO Laser Guide Star Facility including different approaches for the most efficient splitting of the guidestar lasers, and specifications summary for the final selection of the laser launch telescopes.
The team at the Australian National University’s (ANU) Research School of Astronomy and Astrophysics (RSAA) have developed a design concept for such a miniature version, coined Pocket-GMT. Pocket-GMT is designed to simulate GMT’s segmented primary mirror as well as introduce aberrations and distortions similar to what GMT will experience. This would present an opportunity to optimize the functionality of GMT’s control software and wavefront sensors, and to demonstrate phasing within the laboratory prior to full-scale telescope implementation. Pocket-GMT would also be compatible with later GMT instrument prototypes, thus ensuring its usefulness going into the future.
The GMTIFS beam steering mirror uses piezo-walk actuators and a combination of eddy current sensors and interferometric sensors to achieve this dynamic range and control. While the sensors are rated for cryogenic operation, the actuators are not. We report on the results of prototype testing of single actuators, with the sensors, on the bench and in a cryogenic environment. Specific failures of the system are explained and suspected reasons for them. A modified test jig is used to investigate the option of heating the actuator and we report the improved results. In addition to individual component testing, we built and tested a complete beam steering mirror assembly. Testing was conducted with a point source microscope, however controlling environmental conditions to less than 1 micron was challenging. The assembly testing investigated acquisition accuracy and if there was any un-sensed hysteresis in the system. Finally we present the revised beam steering mirror design based on the outcomes and lessons learnt from this prototyping.
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