Since the beginning of the development of the Gran Telescopio Canarias (GTC), an Adaptive Optics (AO) system was considered necessary to exploit the full diffraction-limited potential of the telescope. The GTC AO system designed during the last years is based on a single deformable mirror 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.
Most of the subsystems have been manufactured and delivered. The upgrade for the operation with a Laser Guide Star (LGS) system has been recently approved. The present status of the GTCAO system, currently in its laboratory integration phase, is summarized in this paper.
We report on the comparison between observations and simulations of a completed 12-month field observation campaign at Observatorio del Teide, Tenerife, using ESO's transportable 20 watt CW Wendelstein laser guide star system. This mission has provided sodium photon return flux measurements of unprecedented detail regarding variation of laser power, polarization and sodium D2b repumping. The Raman fiber laser and projector technology are very similar to that employed in the 4LGSF/AOF laser facility, recently installed and commissioned at the VLT in Paranal. The simulations are based on the open source LGSBloch density matrix simulation package and we find good overall agreement with experimental data.
The 10m Gran Telescopio Canarias (GTC) is currently being installed in the Observatorio del Roque de los Muchachos (ORM) on the island of La Palma. An adaptive optics (AO) system will be installed at one of the Nasmyth foci of the telescope within a year of the telescope being commissioned. The preliminary design of the adaptive optics system is presented here. The system will initially be operated in single-conjugate mode using a natural guide star, but provisions are made for upgrade to dual-conjugate operation and the use of laser guide stars. The main system requirements and the optical and mechanical design solutions are outlined here. It is planned to employ a piezo-stack deformable mirror having approximately 350 actuators and a Shack-Hartmann wavefront sensor. The tip-tilt correction will be provided by the secondary mirror of the GTC which is a lightweighted Beryllium mirror with a drive system capable of fast tip-tilt and chopping. In preparation for dual-conjugate operation we have studied the optimal altitude of the second deformable mirror (the first will be conjugate to the telescope pupil) using numerical simulations and measurements of turbulence obtained at the ORM. We have used the GSC II catalogue to determine sky-coverage for multi-natural guide star wavefront sensing, as required for dual-conjugate operation. In addition we have investigated a novel approach to multi-object wavefront sensing based on curvature sensing.
The vertical distribution of the turbulence limits the field of view of classical adaptive optics due to the anisoplanatism. Multiconjugate adaptive optics (MCAO) uses several deformable mirrors conjugated to different layers in the atmosphere to overcome this effect. In the last few years, many studies and developments have been done regarding the analysis of the turbulence volume, and the choice of the wavefront reconstruction techniques.An extensive study of MCAO modelisation and performance estimation has been done at OAA and ONERA. The developed Monte Carlo codes allow to simulate and investigate many aspects: comparison of turbulence analysis strategies (tomography or layer oriented) and comparison of different reconstruction approaches. For instance in the layer oriented approach, the control for a given deformable mirror can be either deduced from the whole set of wavefront sensor measurements or only using the associated wavefront sensor. Numerical simulations are presented showing the advantages and disadvantages of these different options for several cases depending on the number, geometry and magnitude of the guide stars.
Multiconjugate adaptive optics employing several deformable mirrors conjugated at different altitudes has been proposed in order to extend the size of the corrected field of view [FOV] with respect to the size of the corrected FOV given by a classical adaptive optics system. A three dimensional measurement of the turbulent volume is needed in order to collect the information to command the several deformable mirrors. Given a set of guide stars in the field of view, this can be done both using tomography, in which several wavefront sensors are used, each of them coupled to one of the guide stars, or layer oriented techniques, in which wavefront sensors are coupled to a given layer in the atmosphere, and collect light from the whole set of guide stars. We will call this type of measurements optical layer oriented. This type of measurements can be also obtained combining in a numerical way, tomographic measurements. This hybrid approach is called numerical layer oriented. In order to compare their performance, we present an analytical study of the signal to noise ratio [SNR] in the measurements for the two techniques. Optical layer oriented is shown to be more efficient in the range of faint flux and large number of guide stars, while low detector noise will allow numerical layer oriented schemes to be more efficient in terms of SNR.
Modern Acquisition and Guiding systems provide a range of services with the aim of optimizing telescope performance. These include wavefront sensing for active optics, fast guiding and measurement of seeing. On a segmented-mirror telescope, the Acquisition and Guiding system may also be expected to provide measurements of the piston errors between the primary mirror segments. The Guacamole (GUiding, Acquisition and CAlibration MOduLE) system of the 10 m Gran Telescopio Canarias (GTC) has been designed to provide all of these services. Complete systems will be installed at each of the GTC Nasmyth foci. The requirements of this system are presented and a preliminary design is described.
It is foreseen that the Gran Telescopio Canarias will include an Adaptive Optics (AO) system to provide diffraction-limited images in the near-IR. The preliminary scientific requirements are presented here and are used to determine the basic system specifications. The sky coverage is determined for the cases of natural and laser guide stars. The results of conjugating the deformable mirror to the entrance pupil and to the mean turbulence height are compared. The Gran Telescopio Canarias will employ a segmented primary mirror. This will introduce discontinuous wavefront errors due to possible effects of these errors on the AO performance. The resulting requirements for these errors are compared with those determined by the error budget for long-exposure image quality.
The conceptual design of the Gran Telescopio Canarias (GTC) has been completed. One of the challenges facing the GTC Project is to obtain excellent image quality using a segmented primary mirror. The segmentation will introduce a physical effect that contributes to significant degradation of image quality. The image quality requirement imposes the use of an active optical system to correct figure instabilities of the optical surfaces and part of the unavoidable fabrication figure errors. The active correlation includes the capability of 5-axis motion of the secondary mirror, 3-axis motion of each primary mirror segment, and 6 active degrees of freedom to deform each segment. A mixed strategy of closed- and open-loop control of the active correction will be implemented. This paper discusses the expected wavefront errors of the GTC, how they are corrected by active optics, and the expected image quality performance in FWHM, (theta) 80 and Central Intensity ratio.