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Subsections
Abstract:
First light for the ESO VLT unit telescope 1 occurred during the workshop.
David Silva, head of the ESO user support, brought the news and
associated press release information directly from ESO headquarters in
Garching. Some of the ESO press releases which demonstrate the working of
the VLT UT1 are reprinted here.
VLTs
Omega Centauri is the most luminous globular cluster in our Galaxy. As the
name indicates, it is located in the southern constellation Centaurus and is
therefore observable only from the south.
The image shown (Figure 1) here was obtained with the VLT on May 16, 1998, in red light
(R band), i.e. while the mirror was still uncoated. It is a 10-minute
exposure of the center of Omega Centauri and it demonstrates that the
telescope is able to track continuously with a very high precision and thus
is able to take full advantage of the frequent, very good atmospheric
conditions at Paranal. The images of the stars are very sharp
(full-width-at-half-maximum (FWHM) = 0.43 arcsec) and are perfectly round,
everywhere in the field. This indicates that the tracking was accurate to
better than 0.001 arcsec/sec during this observation.
At a distance of about 17,000 light years, this cluster is barely visible to
the naked eye as a very faint and small cloud. When Omega Centauri is
observed through a telescope, even a small one, it looks like a huge swarm
of numerous stars, bound together by their mutual gravitational attraction.
Most globular clusters in our Galaxy have masses of the order of 100,000
times that of the Sun. With a total mass equal to about 5 million solar
masses, Omega Centauri is by far the most massive of its kind in our Galaxy.
Figure 1:
Omega Centauri Tracking Test
 |
Superb image quality is the prime requirement for the VLT. The VLT should
take full advantage of the exceptionally good ``seeing" conditions of the
Paranal site, i.e. moments of a particularly stable atmosphere above the
site, with a minimum of air turbulence.
In this diagram (FIgure 2), the measured image quality of the VLT UT1 astronomical
images is plotted versus the ``seeing", as measured by the Seeing Monitor, a
small telescope also located on top of the Paranal Mountain.
The dashed line shows the image quality requirement, as specified for the
VLT at First Light. The dotted line shows the specification for the image
quality, three years after First Light, when the VLT will be fully
optimized. The fully drawn line represents the physical limit, when no
further image distortion is added by the telescope to that introduced by the
atmosphere.
The diagram demonstrates that First Light specifications have been fully
achieved and, impressively, that the actual VLT performance is sometimes
already within the more stringent specifications expected to be fulfilled
only three years from now.
Various effects contribute to degrade the image quality of a telescope as
compared to the local seeing, and must be kept to a minimum in order to
achieve the best scientific results. These include imperfections in the
telescope optical mirrors and in the telescope motion to compensate for
Earth rotation during an exposure, as well as air turbulence generated by
the telescope itself. The tight specifications shown in this figure
translate into very stringent requirements concerning the quality of all
optical surfaces, the active control of the 8.2-m mirror, the accuracy of
the telescope motions, and, in the near future, the fast ``tip-tilt"
compensations provided by the secondary mirror, and finally the thermal
control of the telescope and the entire enclosure.
The only way to achieve an image quality that is ``better than that of the
atmosphere" is by the use of Adaptive Optics devices that compensate for the
atmospheric distortions. One such device will be operative on the VLT by the
year 2000, then allowing astronomers to obtain images as sharp as about 0.1
arcsec.
In this diagram, both seeing and telescope image quality are measured as the
full-width-at-half-maximum (FWHM) of the light-intensity profile of a
point-like source. The uncertainty of the measurements is indicated by the
cross in the lower right corner.
Figure 2:
Image Quality of the VLT
 |
Figure 3:
Total Optical Control
 |
The VLT active optics system fully controls the primary and secondary
mirrors. In figure 3, images of stars are shown that were obtained
during tests in which the control system forced the optical elements to
produce three different image aberrations, namely triangular, round
(defocus) and linear (astigmatism). This is done by applying different
forces to each of the 150 individual active actuators on which the 8.2-m
main mirror rests and to the position of the secondary mirror. In the case
of the triangular aberration, the mirror was made to resemble Napoleon's
hat. It is worth noting that the deviation of the mirror from its optimum
shape is only 0.015 millimeters.
The great power of this system is demonstrated by the fact that the
resulting stellar images can take on (nearly) any desired form. The optical
system is also consistently brought back to its optimal form, producing the
sharp images of a real star, shown in the lower row.
The control of the two mirrors is such that no significant aberrations
remain after the corrections are applied. The accuracy of the control of
shape of the primary mirror results in an average error of order 0.00005
millimeters. The telescope is only limited by the Earth's atmosphere. In
space, the optical quality of the mirrors under active control could be
diffraction limited.
Next: Near IR Astronomy with
Up: VLT Status and Instruments
Previous: VLT Status and Instruments
Norbert Pirzkal
1998-07-09