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13 May 2003

Information from the European Southern Observatory

Sharper and Deeper Views with MACAO-VLTI

Part 1 of 2

"First Light" with Powerful Adaptive Optics System for the VLT
Interferometer

On April 18, 2003, a team of engineers from ESO celebrated the
successful accomplishment of "First Light" for the MACAO-VLTI
Adaptive Optics facility on the Very Large Telescope (VLT) at the
Paranal Observatory (Chile). This is the second Adaptive Optics (AO)
system put into operation at this observatory, following the NACO
facility (ESO PR 25/01).

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, i.e., as if they were taken from space.

The acronym "MACAO" stands for "Multi Application Curvature
Adaptive Optics" which refers to the particular way optical
corrections are made which "eliminate" the blurring effect of
atmospheric turbulence. 

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 Coude rooms). These systems
correct the distortions of the light beams from the large telescopes
(induced by the atmospheric turbulence) before they are directed
towards the common focus at the VLT Interferometer (VLTI).

The installation of the four MACAO-VLTI units 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.

Put in simple words, with MACAO-VLTI it will become possible to
observe celestial objects 100 times fainter than now. Soon the
astronomers will be thus able to obtain interference fringes with the
VLTI (ESO PR 23/01) of a large number of objects hitherto out of
reach with this powerful observing technique, e.g. 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.

During the present period, the first of the four MACAO-VLTI facilties
was installed, integrated and tested by means of a series of
observations. For these tests, an infrared camera was specially
developed which allowed a detailed evaluation of the performance. It
also provided some first, spectacular views of various celestial
objects, three of which are shown, together with the full text of
this ESO Press Release and all related links, at:

http://www.eso.org/outreach/press-rel/pr-2003/pr-11-03.html

MACAO - the Multi Application Curvature Adaptive Optics facility

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 ESO Multi Application Curvature Adaptive Optics (MACAO) system
uses a 60-element bimorph deformable mirror (DM) and a 60-element
curvature wavefront sensor, with a "heartbeat" of 350 Hz (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 microns. 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 could only be obtained if
the telescope were placed above the Earth's atmosphere.

The technical development of MACAO-VLTI in its present form was begun
in 1999 and with project reviews at 6 months' intervals, the project
quickly reached cruising speed. The effective design is the result of
a very fruitful collaboration between the AO department at ESO and
European industry which contributed with the diligent 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.

Installation at Paranal

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 (formerly UT2).

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 of the actual observations.

AO to the service of Interferometry

The VLT Interferometer (VLTI) combines starlight captured by two or
more 8.2- VLT Unit Telescopes (later also from four moveable1.8-m
Auxiliary Telescopes) and allows to vastly increase the 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, etc., cf.  ESO PR 22/02.

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 microns (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.

 - Continued -

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