MACAO-VLTI
A Curvature AO system for the VLTI

MACAO, which stands for Multi-Application Curvature Adaptive Optics, is an ESO in-house developed 60 elements curvature adaptive optics system. MACAO-VLTI is the application of this AO principle to be used by the VLT interferometer (VLTI). Four MACAO-VLTI systems are to be installed at the each UT Coude' focii feeding the VLTI delay lines with a corrected IR beam from 1000-13000nm with up to 50% Strehl @ 2.2microns. The first MACAO-VLTI unit has been delivered in April 2003 and the final unit will be installed by early 2004.

Built by:	ESO MACAO-VLTI Team

Project Manager:	Robin Arsenault (rarsenau@eso.org)
Instrument Definition	Norbert Hubin (nhubin@eso.org)
Location:	Coude focus of UTs 1-4
Status:	Integration & Commissioning at the Telescopes

MACAO-VLTI Description
MACAO-VLTI Performance
MACAO-VLTI Architecture
MACAO-VLTI On Sky Results
MACAO-VLTI Schedule
MACAO-VLTI Team

MACAO-VLTI Description

The MACAO-VLTI system is a 60 element curvature AO system operating at the Coudé focus of the VLT UTs delivering a corrected wavefront to the VLTI delay lines after reflection off the dichroic M9. The corrective optics consist of a bimorph deformable mirror with 60 actuators mounted in a tip/tilt stage located at M8 in the Coudé train. The Curvature Wavefront Sensor is based on a 60 element lenslet array feeding optical fibers connected to 60 APD modules; the CWFS is hosted in a WFS structure mounted on an X-Y table and sitting on the Coudé platform below M9. MACAO-VLTI is designed to operate only with natural guide stars. For bright sources (mv 8th mag) MACAO is specified to deliver at least 50% Strehl ratio @2.2um on axis under median seeing conditions (0.65") and for faint sources (mV 15.5th mag) at least 25% Strehl ratio @2.2um under the same seeing conditions. In April'03 MACAO-VLTI delivered images with up to 55% Strehl ratio on brigth sources (V=9.9) in worse seeing conditions (0.8"). An IR output beam stability of 10mas ignoring residual refraction and star position errors is guaranteed.

MACAO can close the AO loop on point sources and extended objects with a size < 2.5", up to a maximum of seeing 1.0" @0.5um. The MACAO system can operate with the NGS on-axis or, by tracking the X-Y table on which the CWFS box sits, up to 1' off-axis.

MACAO includes a viewing camera within the WFS box, which allows the performance of the system to be monitored during daytime operations using an artificial source and may also be used as a 10" diameter FOV acquisition camera for the VLTI.

The MACAO software provides automatic guide star acquisition and AO loop optimisation procedures. During closed loop operation it provides low frequency diagnostics via the OLDB and high frequency diagnostics via the VLTI reflective memory network. At the end of an observation a FITS file containing time tagged data characterising the AO correction during the observation is made available to the VLTI.

For the VLTI, up to four Coudé focii will be equipped with MACAO systems which are coordinated with the other subsystems of the VLTI and the TCS by the VLT interferometer supervisor software (VLT ISS) to perform interferometric observations.

MACAO-VLTI Performance

The MACAO-VLTI Performance has been estimated using a MatLab simulation of the system. This simulation was run with a 3 layer atmospheric model (given below) matching what is expected at Paranal.

Layer Wind speed Direction Weight

Upper Layer (10000m) 33m/s Horizontal 0.2

Medium Layer (1000m) 5.7m/s Vertical 0.6

Low Layer (10m) 5.7m/s Horizontal 0.2

The simulation was run for a number of guide star magnitudesand for a number of different seeing conditions. At varous points in the simulation, the PSF is plotted:

And a plot of the resultant Strehl versus Guide Star magnitude obtained.

The plot shown above shows the MACAO-VLTI system performance at 0.65" seeing including the error budget (i.e. losses in optics, misalignment errors etc. The blue curve represents the computer simulated data, the brown curve the measured data in the laboratory. The 2 red circles are performance on the sky for worse seeing (0.8") while the crosses are the specifications issued by VLTI.

MACAO-VLTI Architecture

The following diagram shows a schematic layout of the MACAO-VLTI components in the Coude' lab' and the location of the deformable mirror in the Coude' tube. An overview of the VLT showing the location of the Coude' focus can be found in this diagram.

The subsystems comprising MACAO are:

For each equipped UT

Corrective Optics Assembly

A 60 actuators bimorph deformable mirror mounted in a gimbal Tip/Tilt stage located at M8 in the Coudé tube, driven by HV electronics located on the azimuth platform

Curvature Wavefront Sensor

A Curvature Wavefront Sensing "box" containing the membrane, relay optics and lenslet array connected by a fiber guide to a cooled rack containing the APD modules.

The CWFS box is mounted below M9 and telecentricity lens with the membrane located at the Coudé focus. The box is supported on an X-Y stage for off axis operation.

Control Electronics

Two standard PowerPC based Local Control Units (LCUs) located in cooled cabinets on the Coudé floor. One ICS LCU controlling the X/Y stage and other motor functions, one Real Time Computer LCU containing two PowerPCs performing the real-time adaptive optics control.

Monitoring X-Terminal

Located in the VLTI control room, displaying the diagnostic and monitoring GUIs, the software is hosted on the MACAO instrument workstation.

MACAO-VLTI Corrective Optics

The MACAO corrective optics is a 60 element bimorpph deformable mirror supplied by CILAS mounted in a Tip/Tilt mount supplied by Observatoire de Paris. This unit is located at M8 in the Coude train . The following figure shows the deformable mirror in its Tip/Tilt stage and the whole assembly ready to be mounted in the Coude'.

The MACAO DM has a pupil diameter of 99,7mm but the full diameter is 150mm allowing for the so-called "tilt" electrodes (brownish external ring). The ~3mm thick piezo bimorph has a resonant frequency of ~700Hz and is driven with voltages in the range +/-400V which should provide a sufficient stroke/curvature to correct a 1" seeing.

The Tip/Tilt stage in which the mirror is mounted allows the correction of up to +/-3". The mount is designed such that the center of rotation of the mirror is on the mirror surface, this is essential in an interferometric application to avoid introducing piston when correcting tip/tilt.

MACAO-VLTI Wavefront Sensor Box

Not shown here is the X-Y table on which the Wavefont Sensor Box is hosted.

The WFS box contains:

A safety shutter at the WFS box entrance (1)

The curvature wavefront sensor (CWFS) comprising a variable curvature membrane (5) mounted on a focus (6) and tip/tilt stage (7), relay optics (8) and the CWFS lenslet array (10) connected by fibers to the APD rack
A Neutral Density wheel (9) with 8 positions (one blocking, one free and 6 NDs) situated before the CWFS lenslet array allowing the APDs to operate in their nominal range of < 2Mcounts/sec up to the maximum GS magnitude M_v0.
A rotation unit holding a prism to compensate for the differential rotation between DM and lenslet (11).
A viewing camera (14) allowing the performance of the AO system to be monitored during daytime using the artificial source and during night-time for bright, m_v < 8 NGS. This camera can also operate as a VLTI acquisition camera.

When used as an acquisition camera an objective can be placed in front of the TCCD providing a 10" FOV.

A STRAP Tip/Tilt sensor (13)
A linear stage with 5 positions selecting the beam path within the WFS (3,4)

Open, passing the beam to the CWFS
50/50 beam splitter, sharing the beam between the TCCD with and the CWFS
Folding flat, feeding the STRAP unit
Folding flat, feeding the STRAP unit with a dichroic feeding TBD% of the light to the TCCD
Folding flat, feeding the TCCD via an objective giving a 10" FOV on the TCCD

MACAO-VLTI CWFS Lenslet Geometry

MACAO-VLTI on sky Results

Strehl Ratio and PSF

This is a K-band image of a bright star (V~10) obtained in average seeing conditions (0.8"). Three diffraction rings can clearly be seen with a Strehl ratio larger than 50% and a FWHM of 60 mas. The 3D plot shows the tremendous gain in central intensity illustrating the capability of the AO to "concentrate" the light. The extended seeing limited light is concentrated in a diffraction peak (theoretical limit of the telescope) meaning a ~100 gain in limiting magnitude for the VLTI.

Resolution Improvement

The first image was taken without AO (seeing-limited regime) while the second with the MACAO-VLTI in closed-loop (diffraction-limited). One can see the image improvement to the point where the initial "blob" is resolved and turns out to be a double star. This is a K-band images of a V = 10 star and the separation of the binary is 0.12" and the seeing at the time of observation was ~0.75".

The above is a K-band image of the starburst cluster NGC 3603. MACAO-VLTI was compensating atmospheric disturbances by analyzing light from a star which was 30" away from the field center. The stellar images exhibit Full-Width-Half-Maximum (FWHM) diameter of 0.1 arcsec. The field measures 9 x 9 arcsec.

MACAO-VLTI Schedule

Phase Date Notes

MACAO-VLTI CDR 24 March 2000 Completed

MACAO-VLTI FDR 30 March 2001 Completed

Unit #1 Technical Acceptance Europe Feb 2003 Completed

Unit #1 Release to Observatory Apr 2003 Completed

Unit #4 Technical Acceptance Europe Summer 2004

Unit #4Release to Observatory End 2004

MACAO-VLTI Team

Project Manager Robin Arsenault rarsenau@eso.org

Mechanics Enzo Brunneto
Marco Quattri
Christophe Dupuy ebrunneto@eso.org
mquattri@eso.org
cdupuy@eso.org

Optics Bernard Delabre bdelabre@eso.org

Electronics S. Rossi,
Jaime Alonso srossi@eso.org
jalonso@eso.org

Software Robert Donaldson,
Enrico Fedrigo rdonalds@eso.org
efedrigo@eso.org

System Tests Markus Kasper,
Liviu Ivanescu,
Sylvain Oberti mkasper@eso.org
livanesc@eso.org
soberti@eso.org

Testbench / IR Camera Sebastien Tordo
Jerome Paufique stordo@eso.org
jpaufiqu@eso.org

Integration Christophe Dupuy cdupuy@eso.org

Layer	Wind speed	Direction	Weight
Upper Layer (10000m)	33m/s	Horizontal	0.2
Medium Layer (1000m)	5.7m/s	Vertical	0.6
Low Layer (10m)	5.7m/s	Horizontal	0.2

Phase	Date	Notes

MACAO-VLTI CDR	24 March 2000	Completed
MACAO-VLTI FDR	30 March 2001	Completed
Unit #1 Technical Acceptance Europe	Feb 2003	Completed
Unit #1 Release to Observatory	Apr 2003	Completed
Unit #4 Technical Acceptance Europe	Summer 2004
Unit #4Release to Observatory	End 2004

Project Manager	Robin Arsenault	rarsenau@eso.org

Mechanics	Enzo Brunneto Marco Quattri Christophe Dupuy	ebrunneto@eso.org mquattri@eso.org cdupuy@eso.org
Optics	Bernard Delabre	bdelabre@eso.org
Electronics	S. Rossi, Jaime Alonso	srossi@eso.org jalonso@eso.org
Software	Robert Donaldson, Enrico Fedrigo	rdonalds@eso.org efedrigo@eso.org
System Tests	Markus Kasper, Liviu Ivanescu, Sylvain Oberti	mkasper@eso.org livanesc@eso.org soberti@eso.org
Testbench / IR Camera	Sebastien Tordo Jerome Paufique	stordo@eso.org jpaufiqu@eso.org
Integration	Christophe Dupuy	cdupuy@eso.org

MACAO-VLTI A Curvature AO system for the VLTI

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