Didier Rabaud

NAOS, the first AO system of the VLT: on-sky performance

NAOS is the first adaptive optics system installed at the VLT 8m telescopes. It was designed, manufactured and tested by a french Consortium under an ESO contract, to provide compensated images to the high angular resolution IR spectro-imaging camera (CONICA) in the 1 to 5 μm spectral range. It is equipped with a 185 actuator deformable mirror, a tip/tilt mirror and two wavefront sensors, one in the visible and one in the near IR spectral range. It has been installed in November at the Nasmyth focus B of the VLT UT4. During the first light run in December 2001, NAOS has delivered a Strehl ratio of 50 under average seeing conditions for bright guide stars. The diffraction limit of the telescope has been achieved at 2.2 μm. The closed loop operation has been very robust under bad seeing conditions. It was also possible to obtain a substantial correction with mV=17.6 and mK=13.1 reference stars. The on-sky acceptance tests of NAOS-CONICA were completed in May 2002 and the instrument will be made available to the European astronomical community in October by ESO. This paper describes the system and present the on-sky performance in terms of Strehl ratio, seeing conditions and guide star magnitude.

Status of the VLT Nasmyth adaptive optics system (NAOS)

NAOS is the adaptive optics system to be installed at one of the Nasmyth focus of the VLT. It was designed and manufactured by a French Consortium to provide compensated images to the high angular resolution IR spectro-imaging camera (CONICA) in the 1 to 5 micrometer spectral range. For bright sources, NAOS will achieve a Strehl ratio of 70% under average seeing conditions. It is equipped with a 185 actuator deformable mirror, a tip/tilt mirror and two wavefront sensors, one in the visible and one in the near IR. All the components of NAOS have been delivered and the integration phase is in progress since the beginning of 2000. After extensive tests and performance verifications in France, the system will be shipped to Chile by the end of 2000. The first light at the VLT is foreseen in the beginning of 2001.

Design of the Nasmyth adaptive optics system (NAOS) of the VLT

NAOS is the adaptive optics system to be installed at one of the Nasmyth foci of the very large telescope (VLT). It will provide compensated image to the high angular resolution IR spectro-imaging camera which covers the 1-5 micrometers spectral bands. our French consortium is the sub-contractor of ESO for the design, manufacturing, integration and test of NAOS. For bright sources, the specification is to reach 70 percent Strehl ratio under average seeing conditions. Two wavefront sensors, one in the visible spectral range and one in the near IR spectral range, will equip the adaptive optics system. We foresee to obtained the first light at the VLT unit telescope 1 in mid-2000.

MCAO for Gemini South

The multi-conjugate adaptive optics (MCAO) system design for the Gemini-South 8-meter telescope will provide near-diffraction-limited, highly uniform atmospheric turbulence compensation at near-infrared wavelengths over a 2 arc minute diameter field-of-view. The design includes three deformable mirrors optically conjugate to ranges of 0, 4.5, and 9.0 kilometers with 349, 468, and 208 actuators, five 10-Watt-class sodium laser guide stars (LGSs) projected from a laser launch telescope located behind the Gemini secondary mirror, five Shack-Hartmann LGS wavefront sensors of order 16 by 16, and three tip/tilt natural guide star (NGS) wavefront sensors to measure tip/tilt and tilt anisoplanatism wavefront errors. The WFS sampling rate is 800 Hz. This paper provides a brief overview of sample science applications and performance estimates for the Gemini South MCAO system, together with a summary of the performance requirements and/or design status of the principal subsystems. These include the adaptive optics module (AOM), the laser system (LS), the beam transfer optics (BTO) and laser launch telescope (LLT), the real time control (RTC) system, and the aircraft safety system (SALSA).

First laboratory demonstration of closed-loop Kalman based optimal control for vibration filtering and simplified MCAO

Classic Adaptive Optics (AO) is now successfully implemented on a growing number of ground-based imaging systems. Nevertheless some limitations are still to cope with. First, the AO standard control laws are unable to easily handle vibrations. In the particular case of eXtreme AO (XAO), which requires a highly efficient AO, these vibrations can thus be much penalizing. We have previously shown that a Kalman based control law can provide both an efficient correction of the turbulence and a strong vibration filtering. Second, anisoplanatism effects lead to a small corrected field of view. Multi-Conjugate AO (MCAO) is a promising concept that should increase significantly this field of view. We have shown numerically that MCAO correction can be highly improved by optimal control based on a Kalman filter. This article presents the first laboratory demonstration of these two concepts. We use a classic AO bench available at Onera with a deformable mirror (DM) in the pupil and a Shack-Hartmann Wave Front Sensor (WFS) pointing at an on-axis guide-star. The turbulence is produced by a rotating phase screen in altitude. First, this AO configuration is used to validate the ability of our control approach to filter out system vibrations and improve the overall performance of the AO closed-loop, compared to classic controllers. The consequences on the RTC design of an XAO system is discussed. Then, we optimize the correction for an off-axis star although the WFS still points at the on-axis star. This Off-Axis AO (OAAO) can be seen as a first step towards MCAO or Multi-Object AO in a simplified configuration. It proves the ability of our control law to estimate the turbulence in altitude and correct in the direction of interest. We describe the off-axis correction tests performed in a dynamic mode (closed-loop) using our Kalman based control. We present the evolution of the off-axis correction according to the angular separation between the stars. A highly significant improvement in performance is demonstrated.

Off-axis adaptive optics with optimal control : experimental and numerical validation

We present a laboratory demonstration of open loop Off-Axis Adaptive Optics with optimal control. The control based on a Minimum Mean Square Error Estimator brings a noticeable performance improvement. The next step will be to close the Off-Axis Adaptive Optics loop with a Kalman based optimal control. While this last experiment is currently under progress, a classic Adaptive Optics loop has already been closed recently with a Kalman based control and experimental results are presented. We also describe the expectable performance of the Kalman based off-axis closed loop thanks to an end-to-end simulator. Last minute notice: the Kalman based Off-Axis Adaptive Optics loop has been closed and very first results are given.

ADONIS: a user-friendly adaptive optics system for the ESO 3.6-m telescope

ADONIS (ADaptive OPtics Near Infrared System) is an upgrade of the COME-ON+ adaptive optics prototype. It will allow the astronomical community to use adaptive optics as a common user instrument. This paper describes the main features of the new system, including a mechanical and optical interface for specific visitor equipment and imaging capabilities. We present here the 128 X 128 infrared imaging camera, covering the 1 - 5 micrometers spectral range.

NAOS visible wavefront sensor

This paper describes the Visible Wave Front Sensor (visible WFS) for the VLT Nasmith Adaptive Optics system (NAOS). This Shack-Hartman-based wave front sensor instrument includes within a continuous flow liquid nitrogen cryostat: (1) a low noise fast readout CCD camera controlled by the ESO new generation CCD controller FIERA. The readout noise of this system is 3 e- at 50 kilopixel/sec/port, and is only limited by the CCD intrinsic noise. FIERA proposes remotely controlled readout modes with optional binning, windowing and flexible integration time. (2) two remotely exchangeable micro-lens arrays focusing the analyzed wave front directly on the CCD sensitive surface. The wave front sensor includes also its own atmospheric dispersion compensator. Due to the continuous rotation of the NAOS adapter, the mechanical stiffness of the visible wave front sensor must be very high not to disturb the loop operation (no more than 0.1 micrometer of lenslet array displacement compared to the CCD location over a 30 degree rotation angle of the instrument). The following simulations and tests are described: (1) simulation results providing an estimation of the NAOS maximum operating magnitude, (2) camera optimization, (3) mechanical stiffness measurements.

NAOS performance characterization and turbulence parameters estimation using closed-loop data

An on-line estimation of turbulence parameters (r0, L0 and wind speed) and Adaptive Optics (AO) performance using NAOS [Nasmyth Adaptive Optics System] is presented. The method is based on the reconstruction of open-loop data from deformable mirror voltages and residual wavefront sensor slopes obtained in closed loop. This dedicated tool implemented in the real time computer of the NAOS system (first AO of the Very Large Telescope) allows without any loop opening to automatically monitor and display (every 15 seconds) both the atmospheric conditions and the system performance. We have validated the algorithm and tested its robustness on simulated and experimental data (both in laboratory and on sky). Using data obtained during more than two years of operations, statistical study on NAOS performance and turbulence characteristics are proposed. An on-line estimation of turbulence parameters (r0, L0 and wind speed) and Adaptive Optics (AO) performance is presented.

NAOS real-time computer for optimized closed-loop and online performance estimation

The Real Time Computer RTC is a key component of the Nasmyth Adaptive Optics System, controlling the 185 actuators of the deformable mirror from a 144 Shack-Hartmann subapertures wavefront sensor at a maximum frequency of 500 Hz. It also provides additional capabilities such as real time optimization of the control loop which is the warranty for NAOS to achieve a very good Strehl Ratio in a broad magnitude range (Mv equals 8 up to 18), on-line turbulence and performance estimations and finally capability to store and process the data necessary to the off-line PSF reconstruction algorithm. This RTC is also designed to be easily upgraded as for Laser Guide Star. Moreover all softwares can be easily adapted to control a curvature sensor as well as the hardware which can be used with the two types of wave front sensors.