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GTC Adaptive Optics

GTCAO is a post focal system situated on the Nasmyth platform B, which corrects the optical beam to feed the scientific instrument placed after it.

Introduction

GTCAO is a post focal system situated on the Nasmyth platform B, which corrects the optical beam to feed the scientific instrument placed after it. GTCAO follows the classic design of an AO system with the use of two identical off-axis parabolas, maintaining the effective focal distance of the telescope. On Day 1, the system will provide a single deformable mirror conjugated to the telescope pupil and will use natural stars (NGS) for wavefront sensing.

The GTCAO system is expected to provide a corrected beam that will achieve a Strehl ratio of 0.65 in K-band with bright guide stars. The size of the transmitted field of view is 1.5 arcmin diameter and the optical layout of the system includes an atmospheric dispersion corrector (ADC) working up to 60º zenith angle in order not to degrade its performance with increasing zenith angle.

A future and planned upgrade will allow the system to operate with a Sodium laser guide star (LGS), and also a provision was made to upgrade to a dual-conjugate system with an additional wavefront corrector and a second deformable mirror conjugated to an altitude of approximately 9.8 Km.

GTCAO employs a wavefront sensor of Shack-Hartmann type with 20x20 subapertures and a deformable mirror with 21x21 actuators. For tip-tilt correction it uses the GTC secondary mirror, which is a lightweight beryllium structure.

The first scientific instrument to use GTCAO will be FRIDA (inFRared Imager and Dissector for Adaptive optics), an integral field spectrograph in the near infrared with imaging capability. (see more at http://www.iac.es/proyectos.php?op1=7&op2=19&id=13&lang=en)

Specifications

The main scientific performance requirements for Day 1 are:

ModeSingle-conjugate correction
Wavelength1.0-2.5 micron, with a goal of 0.8-5 micron
Strehl RatioSR>=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 operationSeeing 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 timeAt least 1 h exposure time on the science instrument
Non-sidereal operationContinuous tracking of objects which act as their own guide source at non-sidereal rates limited only by the telescope tracking
DitheringOffsets of 0.25 arcsec (goal 1.0 arcsec) without interrupting operation
NoddingAbility to keep the loop closed while nodding the telescope at 1 arcsec per second (TBC)
ThroughputThroughput 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 imagesDefocused ghosts: <1e-5 (except dichroic 1e-4)
Focused ghosts: <1e-3 and located within 0.2 arcsec)
UpgradesFacilitate 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

Brief history

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.

3D View of the GTCAO components

3D view of the GTCAO components

Description

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 http://dro.dur.ac.uk/10424/1/10424.pdf?DDC116+DDD25+ 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

LocationNasmyth platform B
Mechanical StructureStatic optical bench on a steel truss support
Optical DesignCollimator-camera using two identical off-axis parabolas Ø270mm in a Fried geometry
Image derotation through a three flat mirror K system
DichroicInfrasil Ø180mm 0.9 – 2.5 µm to instrument
Deformable MirrorCilas Ø154mm piezo stack mirror
21x21 (373 useful) actuators
Conjugated to the telescope pupil
sFPDP and RS-485 interfaces
Atmospheric Dispersion CorrectorRotating Amici prisms
Wavefront SensorShack-Hartmann type
20x20 subapertures (3.5”x3.5” each)
2 arcmin patrol field with 3 axis positioner
WFS CameraOCAM2 240x240 24 micron pixels
1500 frames per second. CameraLink interface
Calibration SystemTelescope simulator with NGS and LGS light sources and field mask
Turbulence simulator with two phase screens
Test CameraXenics Xeva 256x320 30 micron pixels
0.9-1.7 micron wavelength
Real Time ControllerPC 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 ControllerBased on industrial controllers IDM680-EI, Servostar S703 and ADAM with CAN interface
SoftwareIntegrated in GTC control system
C++, Java, Python languages
Linux operating system

Present Status

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.

ComponentStatus
Wavefront Corrector Core (without ADC nor Dichroic)Integrated
DichroicReady for integration
ADCContacting manufacturers
Wavefront SensorIntegrating
Electronics and CablingFinished
Real Time Control SoftwareUnder development
Mechanisms SoftwareUnder development
High Level SoftwarePending
Calibration SystemOptical elements ready for integration
Mechanical elements finishing manufacturing
EnclosureIntegrated
Nasmyth Support StructureReady for manufacturing
Test CameraReady for integration

The Team

The present IAC’s personnel working, partially or full time, for GTCAO are:

Principal Investigator: Víctor J. S. Béjar

Project Manager: Carlos Martín

Project Manager LGS: Marcos Reyes

Optics: Marta Puga, Roberto López

Software: José Marco, Josefina Rosich

Electronics: Luis F. Rodríguez, Miguel Núñez, Óscar Tubio

Mechanics: Fabio Tenegi, Roberto Simoes

Scientífic team: Mª Rosa Zapatero (CAB), Almudena Prieto (IAC), José Acosta (IAC), Alan watson (UNAM), Víctor J. S. Béjar (IAC)

Before the project was hand over to the IAC in 2014, the entire development was carried out by GTC personnel. Nevertheless, during the transition phase and more recently, people from GTC has been collaborating actively with the project.

The GTC personnel working partially or full time for GTCAO since 2014 are:

Project Manager/Contact person: Javier Castro

IP Client: Gianluca Lombardi

Optics/System Engineer: Dolores Bello

Mechanics: Lluis Cavaller

Optics: Germán Prieto

Future Dates

Expected transport to telescope at the end of 2018

Publications

2016. GTC adaptive optics hardware electronics. M. Núñez Cagigal et al. SPIE9909, 990935

2016. Status of GTC adaptive optics: integration in laboratory. M. Reyes García-Talavera et al. SPIE9909, 99091C

2015. Current status of the GTC Adaptive Optics System. V. Bejar. 5th Sci with GTC. México

2013. GTCAO system. Descripción y estado. D. Bello. Encuentro RIA AstroMadrid. Madrid

2013. Characterization of the main components of the GTCAO system: 373 actuators DM and OCam2 Camera. D. Bello et al. 3rd AO4ELT Conference. Italy

2004. Preliminary design and plans for the GTC adaptive optics system. N. Devaney et al. SPIE 5490, p913

LGS extension

The use of AO systems is limited to those areas in the sky where there are bright natural guide stars to be used as reference for turbulence measurement. The laser guide stars (LGS) are used to extend the coverage of an AO system to almost the whole sky, even in areas where there is not a natural bright star. Developing a LGS system for GTCAO extends the use of the system to almost any possible scientific object, to most of the sky and to any high resolution scientific program, making the investment in the telescope much more profitable both economically and scientifically.

The work is starting now with simulations on the system performance, determining requirements, evaluating alternatives and technologies, and establishing a conceptual design.

The following phase will be the preliminary design of all subsystems, namely the laser system, the laser beam transfer optics, the laser launch telescope, the laser wavefront sensor, the security system and the control system (including both electronics hardware and control software).

The laser system is the core of the LGS, the baseline is to use the only Sodium Laser available “off-the-shelf”, the one manufactured by the company TOPTICA. The aim of the laser beam transfer optics is to provide the adequate laser beam divergence, maintain stable the beam and guide it to the launch telescope.

The launch telescope is a small telescope that is mounted either on the outer side of the telescope elevation ring (off-axis launch), or behind the secondary mirror (on-axis launch), to send the laser light into the atmosphere to generate the artificial star at 90 km of height.

The laser wavefront sensor is in charge of measuring the effect of the atmospheric turbulence in the light received from the artificial laser star.

A special safety system is required in the facility to mitigate the risks of operation with high power lasers and to comply with the safety regulations. Finally a control system includes all the electronics required to control the different subsystems and synchronize them with the telescope, and the software for the operation.

Note: This project was partially funded by the European Regional Development Fund (ERDF) under the Programa Operativo de Canarias 2007-2013 Axis 1 Priority Theme 2, under Resolution No. 364 of November 25, 2014 of the Canarian Agency for Research, Innovation and Information Society (ACIISI) and loan pre-funded by the Ministry of Science and Innovation

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