The Natural Guide Star Adaptive Optics system for the Gran Telescopio Canarias (GTC) is in its integration phase, and meanwhile the Laser Guide Star update, which will follow two years later, has recently passed its Preliminary Design Phase. This LGS Facility will feature a TOPTICA Na laser, and it will open up the scientific possibilities of GTC enlarging the sky coverage of the AO system and allowing to study at high resolution more scientific targets. A trade-off study was undertaken to decide, among other details, the launching position of the laser and the feasibility of a further upgrade to an MCAO system vs technical complexity, cost and maintenance. As part of this study we have analysed the performance of the GTCAO LGS system to ensure that it will fulfil the specifications in all the different scenarios. Complete end-to-end (E2E) simulations have been performed using the versatile Durham AO Simulation Platform (DASP), including not only real atmospheric profiles from Observatorio del Roque de los Muchachos but also the measured windshake spectrum of the secondary mirror of GTC, the different control loops (TT, DM, focus), the laser uplink jitter and launching telescope divergence, the segmented primary mirror and it's cophasing residual errors, the rotating pupil etc... In this contribution we present a detailed error budget of the system and the results of the E2E simulations that show the impact that such a system will have on the science done with GTC.
The GTC AO system designed and developed during the last years is based on a single deformable mirror with 21 x 21 actuators, conjugated to the telescope pupil, and a Shack-Hartmann wavefront sensor with 20 x 20 subapertures, using an OCAM2 camera. The GTCAO system will provide a corrected beam with a Strehl Ratio (SR) of 0.65 in K-band with bright natural guide stars. This paper reports the updated status of the integration of GTCAO in the IAC laboratory, and the results obtained in the first tests carried out to evaluate the performance of the system, before the complete characterization and the verification of the requirements. The wavefront sensor verification has been completed, and it has been integrated in the optical bench together with the corrector optics including the CILAS deformable mirror. The calibration system, also mounted on the optical bench, includes light sources used to tune, characterise and calibrate the whole system. It also simulates the atmospheric turbulence and the telescope, delivering an aberrated wavefront used to debug the whole control system, and to test the real time control software, the servo loop and different servo control strategies. Finally the Test Camera has been also integrated at the science focus to evaluate the performance.
The Gran Telescopio Canarias Adaptive Optics (GTCAO) will measure the wavefront with a Shack-Hartmann sensor. This wavefront sensor (WFS) is based on the CCD220, an electron-multiplying CCD (EMCCD) that achieves sub-electron readout noise, increasing the signal to noise ratio when weak natural guide stars (NGS) are used as reference. GTCAO will start its operation in telescope with NGS, using only one wavefront sensor, and later it will incorporate a Laser Guide Star (LGS) and consequently a second WFS, also based on an EMCCD. Both EMCCDs and a third one used as spare, have been characterized and compared including the system gain, electron- multiplication gain, readout noise vs gain, excess noise and linearity. The EM gain calibration is important to keep all EMCCD channels in the linear regime and the camera manufacturer carries it out, but it is reported that the multiplication gain may suffer ageing and degradation even if the camera is not in use. This suggests the need to monitor this ageing. In this paper it is proposed and tested a procedure for predictive maintenance that re-characterize the system gain, electron- multiplication gain and linearity periodically in order to predict the eventual ageing of the EMCCD multiplying registers. This procedure can be carried out quickly while the detector is installed in the WFS and in operational status. In order to provide the required illumination, the GTCAO calibration system is used.
The Gran Telescopio Canarias Adaptive Optics (GTCAO) is a single-conjugated post-focal system with a Shack Hartmann wavefront sensor, and one Deformable Mirror (DM) conjugated to the pupil. The optical design for tip-tilt correction includes two different mirrors, DM and the telescope M2, being M2 also used for off-loading the DM to avoid reaching its stroke limits. This optical configuration is open to different control strategies that have been simulated with Matlab. Later it has also been simulated using Durham Adaptive optics Real-time Controller (DARC) and its AO simulator, DASP. Finally some preliminary laboratory results are presented.
The Gran Telescopio Canarias Adaptive Optics (GTCAO) is a single-conjugated post-focal system with a Shack Hartmann wavefront sensor working at visible wavelength and one Deformable Mirror (DM) conjugated to the pupil. GTCAO does not include a fast tip-tilt mirror in its optical bench so it relies on the telescope secondary mirror (M2) to correct low frequency tip-tilt and offload the DM. This paper describes specific details of the software implementation of the mirror control for GTCAO, analyses its computational needs, presents the series of tests performed on the newly designed AO closed loop, and summarises software optimizations and operating system configurations set in order to optimise computer performance in the available hardware architecture
This contribution is focused on the innovative aspects of the design of the Laser Guide Star (LGS) Facility for the Gran Telescopio Canarias (GTC) Adaptive Optics (GTCAO) System . After a trade-off process considering different alternatives, a preliminary opto-mechanical design was defined, based on a “TOPTICA SodiumStar” laser to be launched on-axis. To maximize throughput, different novelties around the optical, and mechanical design of the Laser Launch System, including the Laser Head, the Beam Transfer Optics and the Launch Telescope are emphasized in this paper. In particular, all the elements of the Laser Launch System have been compacted to be placed at the backside envelope of the GTC M2 mechatronics. To fit in that envelope the thermal enclosure of the Laser Head had to be redefined to avoid mechanical interferences and science beam vignetting. An innovative closed-loop Laser Head cooling approach was defined to be also arranged at the backside of GTC M2. Performance simulations running in parallel to the on-axis LGS design could not determine any difference in performance between the on-axis and the off-axis launch. Hence, considering the higher packaging and maintenance complexity required by the on-axis launch, GTC decided to define the off-axis configuration as the new baseline approach. All the solutions already defined for the on-axis approach that were applicable to the new off-axis baseline were reused. To reduce the cost of future upgrades, the LGS design allows generating and launching several LGS with just one launch telescope splitting the light from the Laser Head. In parallel with keeping the volume of the facility to a minimum, an effort to keep its maintenance as simple as possible has been also made to avoid the impact on the telescope operational costs.