Conferences

NAOS - Nasmyth Adaptive Optics System

Results

Laboratory Test Results

Image with Turbulence seeing* = 0.6 arcsec

AO Corrected Image SR = 69 %

Image without Turbulence SR = 90 %

* seeing: strengh of the turbulence

The final integration and test phase took place at CNRS Bellevue in Meudon (France) in 2001. The system was thoroughly tested by the Consortium in laboratory before its shipment to Chile in October 2001. A dedicated optical set-up simulated stars of different magnitudes, atmospheric turbulence with rotating phase screens and F/15 Nasmyth optical beam. Performance validations were performed in these well-defined observing conditions, recording images with CONICA in the different filters.

25/11/01 - First Light

Magnitude of the star = 8
CONICA's imaging wavelength:
= 2.2 µm
Left: uncorrected - Right: corrected

Uncorrected Image FWHM: 0.50''

AO Corrected Image FWHM: 0.07''

* FWHM : Full Width Half Maximum

The on-site re-integration and the installation of NAOS and CONICA started end of October 2001 and lasted one month. The NAOS and CONICA Commissioning was performed through four periods during Moon nights between November 2001 and May 2002 at Paranal. The first light and first compensated image were obtained on the 25th of November. Thanks to the tests made in loboratory, the first compensated image was obtained only one hour after commisioning begining.

Strehl Ratio (SR) as a function of Magnitude (on-sky performance)

The relationship between the phase aberration and image quality is quantified, in a high quality imaging system, by the Strehl ratio , defined as the ratio of the central intensities of the aberrated point spread function and the diffraction-limited point spread function:

where  is the intensity point spread function, and  are image plane coordinates.

A 100% Strehl ratio corresponds to a picture corrected of all the optical aberrations.

Visible Wavefront Sensor

Infrared Wavefront Sensor

On-sky measured SR at 2.2µm obtained with VWFS (top) and IRWFS (bottom)
as a function of NGS photon flux
(equivalent V (visible) and K (infrared) magnitudes respectively).
The shapes of the symbols indicate seeing conditions,
derived from the seeing monitor (dedicated instrument measuring the strengh of the turbulence)
and corrected for airmass:

• stars (good seeing: < 0. 7'')
• cross (moderate seeing < 1.1'')
• diamants (poor seeing > 1.1'')

• grey symbols:
  • large: use of 14x14 lenslet arrays
  • small: use of 7x7 lenslet array

For bright NGS (14xl4 lenslet array), SR varies in a large range between 30% and 60%, i.e. 10 to 15% under laboratory-measured performance.
Bright NGS: SR loss 20 % due to telescope vibrations, differential aberrations
Faint NGS: significant correction, V= 17.5 SR = 8% (seeing 0.75'')

The performance behavior with the increase of the magnitude follows quite well the expected one, down to very faint stars. In this regime, the performance is, as expected, very sensitive to seeing conditions and wind speed.

Correction for very faint Natural Guide Stars (NGS)

SR ~ 6% en K
(gain of a factor 7 in terms of maximum intensity)
Mag V = 17.6, seeing = 0.8'', V-WFS

SR ~ 4% in K
Mag K = 13.1, seeing = 1.2'', IR-WFS

For very faint stars, (V magnitude 17, K magnitude < 13), the system still achieves a significant correction.

Example: Correction for the Moon

	Blurred image of the lunar surface, at = 2.3 microns	Image corrected with NAOS at = 2.3 microns Wavefront sensing on bright lunar peak 20'' south of FoV center (3'' to 4'' diam)

This is the sharpest picture of the Moon ever obtained through ground-based observation.

Performances

The on-sky performance allows to achieve SR more than 60% for good observing conditions and bright NGS in K band. Significant correction is obtained for very faint natural guide star (NGS). The performance is essentially limited by telescope vibrations and, at the time of the tests, was also lirnited by the unavailable calibrations for the differential static aberrations. The calibrations led to a substantial improvement of image quality (10% in J, 4% in K).

In median seeing conditions: SR > 60% at K band

• Substantial compensation with very faint NGS: Mv=17.6, SR=6% (gain of 7)
• Performance limited by telescope vibrations
• Static aberrations pre-compensation: now available
• Fully automated operation (VLT software)
• Many features available in NAOS for specific astronomical observations:

• differential refraction, pointing model
• tracking on moving object,
• chopping, counter-chopping...

Perspectives

• Image deconvolution for residual errors
• 'High Strehl' AO for exo-planet direct imaging
  cf VLT Planet Finder (SR~90%)
• Large Field of View Adaptive Optics correction
  cf Multi-Conjugate Adaptive Optics
• Adaptive optics for extremely large telescopes (ELT)
  (30 to 100m diameter)

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