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VLT First Light

 

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 Tracking Test

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

Image Quality of the VLT

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

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.