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VLT Interferometer Gets Adaptive Optics Feed

Cerro Paranal, Chile – May 13, 2003:   The first MACAO-VLTI adaptive optics system has been successfully installed on one of the Unit Telescopes of the Very Large Telescope (VLT) at the Paranal Observatory in Chile. MACAO-VLTI is intended to feed the VLT Interferometer (VLTI) with a corrected wavefront from the four 8.2 m VLT Unit Telescopes, to improve the efficiency of light injection into the  
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  interferometer's monomode fibres. All four VLT Unit Telescopes will be fitted with MACAO-VLTI AO systems over the next eighteen months, greatly increasing the sensitivity of the VLT Interferometer.

The installation of the four MACAO-VLTI units ("MACAO" stands for "Multi Application Curvature Adaptive Optics"), of which the first one is now in place, will amount to nothing less than a revolution in VLT interferometry. An enormous gain in efficiency will result, because of the associated 100-fold gain in sensitivity of the VLTI.

MACAO-VLTI will make it possible to observe celestial objects 100 times fainter than now. Soon, astronomers will be able to obtain interference fringes with the VLTI of a large number of objects hitherto out of reach with this powerful observing technique, such as external galaxies. The ensuing high-resolution images and spectra will open entirely new perspectives in extragalactic research and also in the studies of many faint objects in our own galaxy, the Milky Way.

Science With Adaptive Optics, Brandner
The achievable image sharpness of a ground-based telescope is normally limited by the effect of atmospheric turbulence. However, with Adaptive Optics (AO) techniques, this major drawback can be overcome so that the telescope produces images that are as sharp as theoretically possible, as if they were taken from space.

About MACAO-VLTI:  
The MACAO-VLTI facility was developed at ESO. It is a highly complex system of which four, one for each 8.2-m VLT Unit Telescope, will be installed below the telescopes (in the Coudé rooms). These systems correct the distortions of the light beams from the large telescopes (induced by atmospheric turbulence) before they are directed towards the common focus at the VLT Interferometer (VLTI).
 
 
Adaptive Optics (AO) systems work by means of a computer-controlled deformable mirror (DM) that counteracts the image distortion induced by atmospheric turbulence. It is based on real-time optical corrections computed from image data obtained by a "wavefront sensor" (a special camera) at very high speed, many hundreds of times each second.

The MACAO system uses a 60-element bimorph deformable mirror (DM) and a 60-element curvature wavefront sensor, which measures wavefront distortions 350 times per second. With this high spatial and temporal correcting power, MACAO is able to nearly restore the theoretically possible ("diffraction-limited") image quality of an 8.2-m VLT Unit Telescope in the near-infrared region of the spectrum, at a wavelength of about 2 µm. The resulting image resolution (sharpness) of the order of 60 milli-arcsec is an improvement by more than a factor of 10 as compared to standard seeing-limited observations. Without the benefit of the AO technique, such image sharpness
MACAO-VLTI A front view of the first MACAO-VLTI unit, now installed at the 8.2-m VLT Kueyen telescope.
  Image:  © ESO
could only be obtained if the telescope were placed above the Earth's atmosphere.

The technical development of MACAO-VLTI started in 1999. The resulting instrument is the outcome of a very fruitful collaboration between the AO department at ESO and European industry, which contributed with the fabrication of numerous high-tech components, including the bimorph DM with 60 actuators, a fast-reaction tip-tilt mount and many others. The assembly, tests and performance-tuning of this complex real-time system was assumed by ESO-Garching staff.

This is the second Adaptive Optics (AO) system put into operation at the Paranal Observatory, following the installation of the NACO facility adaptive optics instrument in 2001.


The Commissioning Work:  
The first crates of the 60+ cubic-meter shipment with MACAO components arrived at the Paranal Observatory on March 12, 2003. Shortly thereafter, ESO engineers and technicians began the painstaking assembly of this complex instrument, below the VLT 8.2 m Kueyen telescope (the UT2 Unit Telescope).

They followed a carefully planned scheme, involving installation of the electronics, water cooling systems, mechanical and optical components. At the end, they performed the demanding optical alignment, delivering a fully assembled instrument one week before the planned first test observations. This extra week provided a very welcome and useful opportunity to perform a multitude of tests and calibrations in preparation for the actual observations.

After one month of installation work and following tests using an artificial light source installed in the Nasmyth focus of Kueyen, MACAO-VLTI had its "First Light" on April 18 2003, when it received "real" light from several astronomical objects.

During the preceding performance tests to measure the image improvement (sharpness, light energy concentration) in near-infrared spectral bands at 1.2 µm, 1.6 µm and 2.2 µm, MACAO-VLTI was checked using a custom-made Infrared Test Camera developed for this purpose by ESO. This intermediate test was required to ensure the proper functioning of MACAO before it is used to feed a corrected beam of light into the VLTI.

After only a few nights of testing and optimizing of the various functions and operational parameters, MACAO-VLTI was ready to be used for astronomical observations. The 3-D plot on the right shows an image taken under average seeing conditions and illustrates the improvement of the image quality when using MACAO-VLTI.


HIC 69495 One of the best images obtained with MACAO-VLTI. The 3-D plot demonstrates the tremendous gain in peak intensity of the AO image (right) as compared to "open-loop" image (the "noise" to the left) obtained without the benefit of AO.
The seeing was 0.8 arcsec at the time of the observations and three diffraction rings can clearly be seen around the star HIC 69495 of visual magnitude 9.9. This pattern is only clearly visible when the image resolution is very close to the theoretical limit. The exposure of the point-like source lasted 100 seconds through a narrow K-band filter. The image has a Strehl ratio (a measure of light concentration) of about 55% and a Full-Width- Half-Maximum (FWHM) of 0.060 arcsec.
  Image:  © ESO
About the VLT Interferometer:  
The VLT Interferometer (VLTI) combines starlight captured by two or more 8.2 m VLT Unit Telescopes (four moveable 1.8-m Auxiliary Telescopes will also be added later) to provide a vast increase in image resolution. The light beams from the telescopes are brought together "in phase" (coherently). Starting out at the primary mirrors, they undergo numerous reflections along their different paths over total distances of several hundred meters before they reach the interferometric Laboratory where they are combined to within a fraction of a wavelength, i.e., within nanometers!

The gain by the interferometric technique is enormous – combining the light beams from two telescopes separated by 100 metres allows observation of details which could otherwise only be resolved by a single telescope with a diameter of 100 metres. Sophisticated data reduction is necessary to interpret interferometric measurements and to deduce important physical parameters of the observed objects, like the diameters of stars.

The VLTI measures the degree of coherence of the combined beams as expressed by the contrast of the observed interferometric fringe pattern. The higher the degree of coherence between the individual beams, the stronger is the measured signal. By removing wavefront aberrations introduced by atmospheric turbulence, the MACAO-VLTI systems enormously increase the efficiency of combining the individual telescope beams.

In the interferometric measurement process, the starlight must be injected into optical fibers which are extremely small in order to accomplish their function; only 6 µm (0.006 mm) in diameter. Without the "refocussing" action of MACAO, only a tiny fraction of the starlight captured by the telescopes can be injected into the fibers and the VLTI would not be working at the peak of efficiency for which it has been designed.

MACAO-VLTI will now allow a gain of a factor 100 in the injected light flux – this will be tested in detail when two VLT Unit Telescopes, both equipped with MACAO-VLTI systems, work together. However, the very good performance actually achieved with the first system makes the engineers very confident that a gain of this order will indeed be reached. This ultimate test will be performed as soon as the second MACAO-VLTI system has been installed later this year.


Reference: 
R. Arsenault, J. Alonso, H. Bonnet, J. Brynnel, B. Delabre, R. Donaldson, C. Dupuy, E. Fedrigo, J. Spyromilio,
T. Erm, J. Farinato, N. Hubin, L. Ivanescu, M. Kasper, S. Oberti, J. Paufique, S. Rossi, S. Tordo, S. Stroebele, J.-L. Lizon, P. Gigan, F. Pouplard, F. Delplancke, A. Silber, M. Quattri and R. Reiss,
"MACAO-VLTI first light: adaptive optics at the service of interferometry," The Messenger 112, 7–12 (2003)
 
 
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