Conferences

NAOS - Nasmyth Adaptive Optics System

Technical Features

NAOS,
the VLT Adaptive Optics System

Caracteristics

• Diameter: 2.5m, Width: 70cm

• Weight at the VLT Nasmyth focus: 1.6 tons of mechanical structure, 600kg of optical and electronic devices

• Weight on the Nasmyth plateform: 500 kg of electronical equipment, 1,5 tons of cable drums and other devices

• Cost: 9 MEuros

Principle

The basic purpose of NAOS is to correct for aberrations in the wavefront due to Earth atmosphere in real time. NAOS is designed to work with natural guide star (NGS), bright or faint stars and extended objects, for wavefront (WF) sensing in both the visible (V) and infrared (IR) spectral range. Provisions are made for future implementation of the laser guide star technology. NAOS compensates for turbulence in order to provide diffraction limited images at 2.2µm on a 8m class telescope to the high angular resolution infrared (IR) spectra imaging instrument (CONICA) covering the 1 - 5µm spectral range, i.e. from J band to M band. Provisions are also made for a second output focus to be equipped with a visitor instrument (possibly a visible camera) in parallel to CONICA.

Under average seeing conditions, i.e. 0.9arcsec seeing, NAOS has demonstrated during the commissioning periods an on-sky correction capability as high as 50% Strehl ratio (SR) when locked on bright NGS.

NAOS Optical Train

View of the NAOS optical train.
VLT Nasmyth focus: F/15, 2arcmin FOV (field of view).
NAOS output focus: F/15, 2arcmin FOV.

On this figure, the wavefront sensor (WFS) channel is not drawn. The NAOS input beam is the VLT F/15 Nasmyth focus beam in a 2arcmin in diameter unvignetted field of view. NAOS transmits a turbulence-compensated F/15 beam in a 2arcmin field of view to the astronomical instruments. Two off-axis parabolas allow to reimage the telescope pupil on the deformable mirror (DM) and the Nasmyth focal plane at the entrance focal plane of CONICA. The tip-tilt mirror (TTM) is plane and is mounted upstream from deformable mirror and close to the pupil plane. A dichroic plate splits the light in transmission toward CONICA (instrument channel) and in reflection toward the wavefront sensor channel. A set of three dichroics and 2 neutral beam-splitters is offered to match the selection of the instrument spectral bands with respect to the choice of the wavefront sensor unit. For the Dichroics, the transmission is very high (~ 92%).

Note that a pick-up mirror (or dichroic) can be set up in front of the CONICA input focus in order to feed the visitor instrument channel. All the mirrors of the optical train have a very high reflectivity in the spectral range between 0.45 and 5.0µm and a good optical quality.

Corrective Optics

The tip-tilt mirror (TTM) compensates for the overall wavefront tilt fluctuations while the higher orders are compensated by the deformable mirror (DM). Real-time corrections are applied both to the surface of the deformable mirror and to the orientation of the tip-tilt mirror.

• The tip-tilt mirror was manufactured by the Observatoire de Paris (ODP). It is made of a two-axis gimbal mount equipped with four voice coil actuators working in push-pull. The mirror itself is 50% light-weighted. Per axis, the angular range is 13arcsec PV (peak-to-valley) on sky with a 2.1marcsec rms resolution. The measured temporal bandwidth of the tip-tilt mirror closed-loop transfer function is 350Hz at -3dB, limited by the first mechanical resonance frequency.


Adaptive mirror:
the two-axis light-weighted tip-tilt mirror
manufactured by ODP (uncoated blank)

• The deformable mirror is a continuons facesheet mirror with 185 active piezo-stacked actuators on a square array in a 110 mm diameter pupil image. The deformable mirror was manufactured by CILAS (France). The total wavefront stroke is 20 µm PV (peak-to-valley). The wavefront error (WFE) at rest is lower than 2.0 µm PV, easily corrected in closed-loop. The non-correctable high spatial frequency wavefront error is lower than 30 nm rms. This mirror has a very low hysteresis and a very high temporal bandwidth.


Adaptive Mirror:
the 185 actuator
deformable mirror
manufactured by CILAS

Deformable Mirror:
piezo-electric actuators
move the reflective diaphragm.

The two Wavefront Sensor (WFS) Units

The WFS optical train incorporates the Field Selector (FS), the WFS selector mirror and the two WFS units. The Field Selector, placed at the entrance of the WFS channel, selects the NGS for wavefront sensing in the 2arcmin Field of View (FOV). In addition, it allows moving guide object tracking, differential refraction, precalibrated flexure compensation and counter-chopping. It is made up of two parallel tip-tilt mirrors (FSI and FS2) working in closed loop to achieve a very high angular stability. The two Field Selector mirrors were manufactured by CSEM (Switzerland).

The two NAOS WFS are Shack-Hartmann based WFS, providing two lenslet array pupil sampling in order to cover a very large magnitude range (See figure below: "Shack-Hartmann Principle "). The 14xl4 subaperture configuration (144 valid subapertures) is well adapted for bright NGS. While the 7x7 configuration (36 valid subapertures) allows to achieve substantial correction with faint NGS.

• The visible WFS (VWFS) operates in the 0.45 - 1µm wavelength range.

• The infrared WFS (IRWFS) operates in the 0.8 - 2.5µm wavelength range.

Shack-Hartmann Principle

Example : Pupil Sampling with the 14x14-WFS Sub-Aperture Configuration

Real Time Computer and Software

The computer hardware architecture of NAOS is built around a workstation and four local control units (LCU) connected together by a local area network (LAN). The workstation is in charge of the execution of the high level softwares of NAOS. Each subsystem is controled by one LCU: IRWFS, the VWFS camera, the electromechanisms of the instrument including the field selector and the real-time computer (RTC). The system is self-configuring to adapt to changing atmospheric conditions, observed objects or types of observations carried out.

NAOS and CONICA

NAOS + CONICA attached to the VLT Nasmyth adapter-rotator

An almost perfect picture is continuously provided to CONICA (COude Near Infrared CAmera) by NAOS.

NAOS is directly attached to the Nasmyth adapter-rotator of the telescope and CONICA is rigidly attached to NAOS. Therefore, both NAOS and CONICA rotate with the adapter-rotator around the altitude axis of the telescope to compensate for field rotation. For CONICA, the NAOS mechanical structure is seen as an adapter attached to the Nasmyth adapter-rotator. The NAOS adapter carries the optical train, including the corrective optics, the two WFS units, the front-end electronicsl and holds CONICA (and a possible visitor instrument). During the rotation of NAOS + CONICA, the gravity load induces flexures of the optical elements inside NAOS and CONICA and of the CONICA flange.

Last Update: July 13, 2004 - © ONERA 2009 - Terms of use