“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.

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’s, 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.

MACAO-VLTI First light

After one month of installation work and following tests by means of
an artificial light source installed in the Nasmyth focus of KUEYEN,
MACAO-VLTI had “First Light” on April 18 when it received “real” light
from several astronomical obejcts.

During the preceding performance tests to measure the image
improvement (sharpness, light energy concentration) in near-infrared
spectral bands at 1.2, 1.6 and 2.2 microns, MACAO-VLTI was checked by
means of 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 images below were taken under
average seeing conditions and illustrate the improvement of the image
quality when using MACAO-VLTI.

MACAO-VLTI – First Images

Here are some of the first images obtained with the test camera at the first
MACAO-VLTI system, now installed at the 8.2-m VLT KUEYEN telescope.

PR Photos 12b-c/03 show the first image in the infrared K-band
(wavelength 2.2 microns) of a star (visual magnitude 10) obtained
without and with image corrections by means of adaptive optics.

PR Photo 12d/03 displays one of the best images obtained with
MACAO-VLTI during the early tests. It shows a Strehl ratio (measure of
light concentration) that fulfills the specifications according to
which MACAO-VLTI was built. This enormous improvement when using AO
techniques is clearly demonstrated in PR Photo 12e/03, with the
uncorrected image profile (left) hardly visible when compared to the
corrected profile (right).

PR Photo 11f/03 demonstrates the correction capabilities of MACAO-VLTI
when using a faint guide star. Tests using different spectral types
showed that the limiting visual magnitude varies between 16 for
early-type B-stars and about 18 for late-type M-stars.

Astronomical Objects seen at the Diffraction Limit

The following examples of MACAO-VLTI observations of two well-known
astronomical objects were obtained in order to provisionally evaluate
the research opportunities now opening with MACAO-VLTI. They may well
be compared with space-based images.

The Galactic Center

The center of our own galaxy is located in the Sagittarius
constellation at a distance of approximately 30,000 light-years. PR
Photo 12h/03 shows a short-exposure infrared view of this region,
obtained by MACAO-VLTI during the early test phase.

Recent AO observations using the NACO facility at the VLT provide
compelling evidence that a supermassive black hole with 2.6 million
solar masses is located at the very center, cf. ESO PR 17/02. This
result, based on astrometric observations of a star orbiting the black
hole and approaching it to within a distance of only 17 light-hours,
would not have been possible without images of diffraction limited
resolution.

Eta Carinae

Eta Carinae (PR Photo 12i/03) is one of the heaviest stars known, with
a mass that probably exceeds 100 solar masses. It is about 4 million
times brighter than the Sun, making it one of the most luminous stars
known.

Such a massive star has a comparatively short lifetime of about 1
million years only and – measured in the cosmic timescale- Eta Carinae
must have formed quite recently. This star is highly unstable and
prone to violent outbursts. They are caused by the very high radiation
pressure at the star’s upper layers, which blows significant portions
of the matter at the “surface” into space during violent eruptions
that may last several years. The last of these outbursts occurred
between 1835 and 1855 and peaked in 1843. Despite its comparaticely
large distance – some 7,500 to 10,000 light-years – Eta Carinae
briefly became the second brightest star in the sky at that time (with
an apparent magnitude -1), only surpassed by Sirius.

Frosty Leo

Frosty Leo (PR Photo 12j/03) is a magnitude 11 (post-AGB) star
surrounded by an envelope of gas, dust, and large amounts of ice
(hence the name). The associated nebula is of “butterfly” shape
(bipolar morphology) and it is one of the best known examples of the
brief transitional phase between two late evolutionary stages,
asymptotic giant branch (AGB) and the subsequent planetary nebulae
(PNe).

For a three-solar-mass object like this one, this phase is believed to
last only a few thousand years, the wink of an eye in the life of the
star. Hence, objects like this one are very rare and Frosty Leo is
one of the nearest and brightest among them.

Contacts

Norbert Hubin
ESO
Garching, Germany
Phone: +49 89 3200-6517
email: nhubin@eso.org

Robin Arsenault
ESO
Garching, Germany
Phone: +49 89 3200-6524
email: rarsenau@eso.org

Markus Kasper
ESO
Garching, Germany
Phone: +49 89 3200-6359
email: mkasper@eso.org