The main components of GTCAO are the wavefront corrector, the wavefront sensor, the calibration system, the test camera, the mechanical structure and the control system.

    Wavefront Corrector
    The wavefront corrector is the part of the AO system where the wavefront aberrations caused by the atmosphere are corrected. Its optical design is based on the widely employed design of a collimator-camera formed using a pair of identical off-axis parabolas, with the deformable mirror placed in the collimated beam conjugate to the pupil. The optical system has unit magnification and preserves the focal ratio of the telescope providing a telecentric output beam. The diameter of the unvignetted FoV is 2.05 arcmin and the image quality exceeds the requirements within the 1.5 arcmin diameter FoV.

    Optical Design of the wavefront corrector

    Optical design of the wavefront corrector

    The elements of the wavefront corrector are:

    Optical derotator. It consists of three flat mirrors in a K configuration mounted on a common structure which rotates around the optical axis in order to compensate the rotation of the sky produced by the alt-azimuth telescope mount. It is situated on the optical table and decoupled from the mechanical structure of the telescope rotator.

    Off-axis parabolas. Two identical mirrors made of Zerodur with Ø270x36mm, 2008.6mm focal length and 636.3mm off-axis distance.

    Fold mirror. Flat Zerodur mirror with Ø220x30mm, conjugated to an altitude of 9.8km and used, on Day 1, to conveniently pack the system.

    Deformable mirror (DM). Manufactured by CILAS, it is a stack array mirror with Ø154mm and 21x21 piezo electric actuators (i.e. 373 useful actuators) spaced ~7mm and with +/-5.5 micron mechanical stroke. It is conjugated to the pupil of the telescope (M2).

    Dichroic. It is an Infrasil substrate plate Ø180mm which transmits the infrared radiation to the scientific instrument, while it reflects the visible radiation to feed the wavefront sensor. Cut-off wavelength 0.9µm-2.5µm.

    Atmospheric Dispersion Corrector (ADC). The ADC is composed of a pair of prisms with Ø155mm which are anti-symmetrically rotated to compensate the atmospheric dispersion as a function of the zenith angle and, as a whole, to orientate the introduced dispersion with the parallactic angle. It is designed to operate in the z-J-H and K bands although optimized in the H band. For those observations not requiring the ADC, it can be removed from the optical beam.

    View of the frontwave corrector under integration at the laboratory

    View of the Wavefront corrector under integration at the laboratory

    Wavefront Sensor
    The wavefront sensor (WFS) is of Shack-Hartmann type. It is designed to operate in a high order wavefront sensor mode with 20x20 subapertures (lenslet array) in a Fried geometry arrangement, and in a low order wavefront sensor mode with 2x2 subapertures. The latter will be employed when operating with LGS as a tip-tilt and defocus sensor on NGS.

    The wavefront sensor is attached to a 3-axis positioner, so it can be placed to pick-off a guide star at any position in the optical field of view (2.0 arcmin).

    3D model view of the WFS

    3D model view of the WFS

    WFS 3-axis positioner (up-right) and opto-mechanical integration in the lab

    WFS 3-axis positioner (up-right) and opto-mechanical integration in the lab

    The optical design of the wavefront sensor is divided in two stages. The first stage, a collimator-camera achromatic lenses relay, is employed to place an ADC at a pupil image, not to produce significant chromatic effects at the lenslet array plane. This first stage includes also (within the collimated beam) a filter wheel and a pupil positioner consisting on a plane parallel plate mounted on a commercial tip-tilt mount. The second stage is a collimator consisting in two doublets that conjugates the pupil plane onto the lenslet arrays. A wheel hosting both 2x2 and 20x20 lenslet arrays, allows the selection of the pupil sampling for both WS modes, keeping the focal plane at the detector plane. Additionally, at the WFS entrance there is an aperture wheel and a LED for calibration purposes. Finally, two fold mirrors are used to compact the system in a “Z” shape.

    The wavefront sensor camera is an OCAM2 with an e2v CCD220 detector, a frame-transfer 8-output back-illuminated sensor using EMCCD technology. It has 240x240 24 micron pixels and a readout speed of 1500 frames per second. The image scale on the camera is 0.35 “/px, giving a usable field of view of 3.5”x3.5” in each lenslet.

    Calibration System
    The purpose of the calibration system is to provide a set of illumination sources to introduce light in the adaptive optics system for calibration. The calibration system is situated before the optical derotator and it consists of two units: the telescope simulator (GTCSim) and the focal plane calibration unit.

    The focal plane calibration unit is composed of a linear table which supports a moving structure that has one position for the field simulator mask and one position for two flat mirrors in a periscope arrangement, allowing feeding the adaptive optics system with the light coming from the telescope simulator. Moving the linear table, it is possible to place on the telescope focal plane the field simulator mask, the light coming from the GTCSim or nothing for normal observation.

    3D model view of the calibration system

    3D model view of the Calibration System

    The field simulator mask is situated at the focal plane of the telescope and it consists of a series of point-like sources where some of them are LEDs and other are holes illuminated by the GTC Instrument Calibration Module Infrared Lamps. The field simulator mask and its light sources are used to calibrate the WFS (FoV and guide star pick-up mechanism), to calibrate the field distortion at the output plane of GTCAO and the iteration matrix.

    The GTCSim is a telescope and turbulence simulator whose task is to provide a beam at the entrance focus of the AO system with the same focal ratio as the nominal beam coming from GTC. The simulator includes a pupil stop with the same shape as the GTC pupil and, in order to simulate the effect of pupil rotation, the optical derotator K-system of the wavefront corrector can be used. The turbulence simulator is intended to allow the performance to be checked in two seeing conditions, at 0.5” and 1.5” seeing. As the wavefront tip-tilt component will be corrected by the GTC secondary mirror (M2), the phase screens only have a small tip-tilt component, corresponding to the expected residual tip-tilt once corrected by M2. There is a mechanism to select the phase screens or to retract them from the optical path.

    Within GTCSim there is a LED and a halogen lamp to simulate, in the visible and infrared, a natural guide star (NGS), i.e. a reference source at an infinite distance to the telescope pupil, and there is also a yellow LED to simulate a laser guide star (LGS), i.e. a reference source at a finite distance to the telescope pupil.

    Non-common path aberration will be calibrated using the NGS light source of GTCSim.

    Test Camera
    The test camera will be used to test and verify the AO system both at the laboratory and at the telescope in the absence of a science instrument. The camera is a Xenics Xeva with 256x320 30 micron pixels working between 0.9 and 1.7 micron wavelength. The design comprises off-the-shelf optical components and a filter wheel. It provides a Nyquist sampling at 1.25 micron.

    View of the test camera at the laboratory

    View of the test camera at the laboratory

    Mechanical Structure
    The GTCAO system is situated on the Nasmyth platform B. It is a static system supported directly by the Nasmyth platform and it doesn’t have any mechanical interface with the Nasmyth instrument rotator. The scientific instrument feed by GTCAO is also attached statically on the Nasmyth platform by its own support structure, so there is no mechanical link between the instrument and the adaptive optics system, other than the Nasmyth platform itself.

    The opto-mechanical components of GTCAO are located on an optical bench supported by a 1200 kg steel truss structure on the Nasmyth platform. The optical bench with all its components installed, including its protective enclosure, weights 1360 kg.

    As the bench is placed at 1.5m over the Nasmyth floor, two access platforms are planned to provide an easy access to the optical elements for checking and maintenance tasks.

    3D view of GTCAO at the Nasmyth platform B

    3D view of GTCAO at the Nasmyth platform B

    Control System
    The adaptive optics control system (AOCS) main tasks consist of controlling the opto-mechanical elements, computing the real time closed-loops, acquiring data from sensors and cameras, and implementing the calibration procedures.

    The AOCS will be integrated in the GTC control system, following the established hardware and software architecture and taken advantage of the services provided by the existing software, e.g. alarm and log processing, configuration, event synchronization, user interface, persistent storage, etc.

    The hardware consists of two PCs, two electronic cabinets and three auxiliary boxes. Both PCs are rack mounted servers. The one dedicated to the real time control (RTC) has 2 Intel Xeon E5-2650V3 10 core CPUs, while the other, in charge of the control of the mechanisms and interfaces with the rest of the system and telescope, is a standard industrial PC, based on an i7 CPU (TBC).

    The PCs and the control electronics of the deformable mirror are located inside one cabinet (control cabinet), while all the drivers and power units to control the mechanisms (IDM680-EI and Servostar S703) are inside the power cabinet. Both cabinets stand on the Nasmyth platform few meters away from the optical bench.

    Diagram of the control hardware of GTCAO

    Diagram of the control hardware of GTCAO

    View of the power cabinet at the laboratory

    View of the power cabinet at the laboratory

    There are three small boxes attached to the optical bench dedicated to the electrical interfaces and components needed to monitor and control the temperatures sensors, illumination sources, entrance shutter and the electronics of the WFS camera.

    The real time control software being evaluated is based on the Durham University adaptive optics real time controller (DARC), a generic adaptive optics control system using off-the-shelf modern and powerful central-processing-units and capable of accepting hardware acceleration (GPUs, FPGAs…). The purpose of the RTC is to read the images coming from the WFS camera and calculate the signals to be send to the deformable mirror in order to compensate the perturbations introduced by the atmosphere on the image. The goal is to perform these calculations at the maximum frequency (1500 times per second) with the minimum possible latency, in order to correct the perturbations before the atmosphere has changed. See for more details about DARC.

    RTC uses a Matrox Radient eCL frame-grabber with a CameraLink interface to read the detector and a dual Curtiss-Wright sFPDP card to command the deformable mirror. There is also a RS-485 line to the DM for housekeeping purposes.

    Summary of technical parameters
    Location Nasmyth platform B
    Mechanical Structure Static optical bench on a steel truss support
    Optical Design Collimator-camera using two identical off-axis parabolas Ø270mm in a Fried geometry
    Image derotation through a three flat mirror K system
    Dichroic Infrasil Ø180mm 0.9 – 2.5 µm to instrument
    Deformable Mirror Cilas Ø154mm piezo stack mirror
    21x21 (373 useful) actuators
    Conjugated to the telescope pupil
    sFPDP and RS-485 interfaces
    Atmospheric Dispersion Corrector Rotating Amici prisms
    Wavefront Sensor Shack-Hartmann type
    20x20 subapertures (3.5”x3.5” each)
    2 arcmin patrol field with 3 axis positioner
    WFS Camera OCAM2 240x240 24 micron pixels
    1500 frames per second. CameraLink interface
    Calibration System Telescope simulator with NGS and LGS light sources and field mask
    Turbulence simulator with two phase screens
    Test Camera Xenics Xeva 256x320 30 micron pixels
    0.9-1.7 micron wavelength
    Real Time Controller PC based with 2 Intel Xeon E5-2650V3 10 core CPUs 64GB RAM 640 GB SSD
    Matrox Radient eCL frame-grabber
    Curtiss-Wright sFPDP card
    Mechanisms Controller Based on industrial controllers IDM680-EI, Servostar S703 and ADAM with CAN interface
    Software Integrated in GTC control system
    C++, Java, Python languages
    Linux operating system

    Mode: Single-conjugate correction
    Wavelength: 1.0-2.5 micron, with a goal of 0.8-5 micron
    Strehl Ratio: SR>=0.65 at 2.2 micron for a bright NGS on axis
    SR>=0.1 at 2.2 micron for a faint NGS (mR=14.5)
    Range of operation Seeing better than 1.5 arcsec FWHM at 500 nm
    Zenith angles 0º - 60º
    Field of View (FoV) 1.5 arcmin available to the science instrument
    1.5 arcmin accessible for wavefront sensing
    Observation time At least 1 h exposure time on the science instrument
    Non-sidereal operation Continuous tracking of objects which act as their own guide source at non-sidereal rates limited only by the telescope tracking
    Dithering Offsets of 0.25 arcsec (goal 1.0 arcsec) without interrupting operation
    Nodding Ability to keep the loop closed while nodding the telescope at 1 arcsec per second (TBC)
    Throughput Throughput of wavefront corrector shall be at least 70% in the wavelength range from 1.0 to 2.5 micron with a goal of 70% in the range 0.8 to 5 micron
    Emissivity < 20% at 3.8 micron
    Ghost images Defocused ghosts: <1e-5 (except dichroic 1e-4)
    Focused ghosts: <1e-3 and located within 0.2 arcsec)
    Upgrades Facilitate upgrade to a multi-conjugate system having two deformable mirrors.
    Facilitate upgrade to the use of a single LGS and upgrade to the use of multiple LGS

    Since the beginning of the design of the GTC, It was clear that in order to exploit the full diffraction-limited potential of the telescope, an Adaptive Optics (AO) system would be necessary, and provisions were made to ensure that the telescope would not limit the future AO performance. Well before the telescope entered its scientific operation, in 2009, the AO system was already in development.

    The AO conceptual design was carried out during 2001, and in 2004 took place the preliminary design review. In January 2008 the project passed the Advance Design Review and started the detailed design and manufacturing. The integration of subsystems began in 2012.

    The GTCAO system was initially developed by the GTC project office but due to a lack of resources in the development team, the project paused in 2013. An agreement between IAC, GTC and the Canary Government was made in order to continue the development and incorporate, as a second phase, a Laser Guide Star facility. The project restarted effectively in 2015.


    Expected transport to telescope at the end of 2018

    GTCAO is in integration phase at the IAC’s laboratory but not all the subsystems are at the same level of development. The following table shows the present status of the main components.

    Component Status
    Wavefront Corrector Core (without ADC nor Dichroic) Integrated
    Dichroic Ready for integration
    ADC Contacting manufacturers
    Wavefront Sensor Integrating
    Electronics and Cabling Finished
    Real Time Control Software Under development
    Mechanisms Software Under development
    High Level Software Pending
    Calibration System Optical elements ready for integration
    Mechanical elements finishing manufacturing
    Enclosure Integrated
    Nasmyth Support Structure Ready for manufacturing
    Test Camera Ready for integration

    Related Projects

    • GTCAO installed at the Nasmyth B platform of GTC telescope
      GTCAO LGS. Adaptative Optics and Laser Guide Star for GTC
      The Adaptive Optics (AO) for the Gran Telescopio Canarias (GTC) corrects the effect of the atmospheric turbulence on the light, to exploit the high spatial resolution capability of GTC. The Laser Guide Star (LGS) extends the coverage of GTCAO to any part of the sky, increasing dramatically the capability to do high spatial resolution science.
      Víctor Javier
      Sánchez Bejar

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