The Adaptive Optics (AO) of the Gran Telescopio Canarias (GTC) is a single conjugate postfocal system, integrated in one of the Nasmyth platforms of the telescope. GTC is located in the Observatory of Roque de Los Muchachos (ORM) in the island of La Palma, Spain. GTCAO is based on a single deformable mirror (DM) with 373 actuators, conjugated to the GTC pupil, and a Shack-Hartmann wavefront sensor (WFS) with 312 useful sub-apertures on an OCAM2 camera. The performance required for GTCAO is 65% Strehl Ratio in K-band under average atmospheric conditions and bright NGS. After finishing its laboratory testing and calibration in 2022, the laboratory acceptance and transport readiness review took place in April 2023. GTCAO integration in the telescope was carried out along June 2023. The GTCAO control software was integrated with the GTC observing software along July, to implement the optical derotation, the WFS positioning in the field and guiding, the WFS atmospheric dispersion compensation, and the tip-tilt correction loop implemented with the GTC secondary mirror. At the end of Summer 2023 started the on-sky commissioning. Since then, the AO loop has been closed on sky in different turbulence and guide star conditions. This paper presents the GTCAO integration results and first on sky commissioning results.
GTCAO is the instrument that implements Adaptive Optics on GTC. For atmospheric turbulence correction, GTCAO uses a deformable mirror. Unlike other AO systems, GTCAO does not include a dedicated mirror for low-frequency tip-tilt (TT) correction. In the absence of dedicated correction, the TT components of atmospheric turbulence are corrected by the deformable mirror (DM), using a significant portion of its working range and potentially leading to saturation. To mitigate this effect, GTCAO calculates low-frequency TT and offloads its correction to the telescope secondary mirror (M2) and primary axes. These actions optimize the use of the deformable mirror range for higher frequencies correction and extends the deformable mirror lifespan. This paper describes the approach implemented for calculating low-frequency TT from the information provided by the Shack-Hartmann wavefront sensor and the state of the DM, in both open-loop and closed-loop operation modes. Laboratory tests and telescope real observation results are also presented.
The Gran Telescopio de Canarias Adaptive Optics System (GTCAO) is currently in its commissioning phase at Roque de Los Muchachos Observatory (ORM). The GTCAO is a single-conjugated post-focal system equipped with a Shack- Hartmann Wavefront Sensor (WFS) and a Deformable Mirror (DM) conjugated to the pupil, achieving a Strehl Ratio of 65% in the K-band by utilizing a natural bright star. By early 2023, the development of the AO system concluded at the facilities of the Instituto de Astrofísica de Canarias (IAC), where acceptance tests were carried out. Subsequently, the entire system was integrated onto the Nasmyth platform of the telescope, replicating the controlled laboratory conditions. During maritime and land transportation, the system was handled with the optics train integrated and high-precision alignment. This involved the development of specific transportation tools to prevent accelerations beyond 2g, which could induce plastic deformations and misalignments in the opto-mechanical components. Extensive vibration analysis and different Power Spectral Densities (PSD) profiles were crucial to meet the requirements. A rigorous integration procedure was devised to ensure safe assembly, spanning four consecutive daytime shifts. This meticulous approach was adopted to guarantee that the telescope’s observing hours remained uncompromised. This article provides a comprehensive account of the integration process and emphasizes the mechanical aspects. It includes static and dynamic mechanical analyses and technical details of handling, transport, and integration from the lab to the telescope to ensure safety and high precision assembly of opto-mechanical components.
The Adaptive Optics system of the 10-m class Gran Telescopio Canarias (GTCAO) is completing the acceptance tests in the laboratory at the Instituto de Astrofísica de Canarias, to be ready for its integration in the telescope at Roque de Los Muchachos Observatory (ORM) in 2023. The AO system has been designed with robustness and operability as its key characteristics, and will be a facility of GTC. It features a single deformable mirror (DM) with 373 actuators, conjugated to the telescope pupil, and a Shack-Hartmann wavefront sensor (WFS) with 312 subapertures, using an OCAM2 camera. The expected performance of the GTCAO system working on average atmospheric conditions and bright NGS is 65% Strehl Ratio in K-band. In this paper we present the characterization of the system and the tests that have been performed for its acceptance at the laboratory. A series of calibrations are required and methodically run to achieve its ultimate performance: flatten the DM, acquire the Interaction Matrix and the reference slopes, correct the non-common path aberrations etc. The WFS requires additional calibrations, to compensate the pupil displacement in all the 2 arcminutes patrol field of view, and to correct the atmospheric dispersion in the visible. Close loop gains and sampling frequency are adapted to the changing conditions, and lookup tables are created for that purpose. The compliance with the system specifications has been verified. After verification of the final software functionalities for telescope operation, the system will be shipped to the ORM, to be installed and aligned on the Nasmyth platform of the GTC, and to be integrated with the telescope control system.
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