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The VLT Sharpens its View

10 Giugno 1998

Following the very successful First Light Event in late May, the first 8.2-m Unit Telescope (UT1) of the ESO Very Large Telescope (VLT) has continued to demonstrate its great potential. These developments are reported on a dedicated webpage with new VLT Information.

The VLT and Paranal - a fine combination

The advanced, computerized system that provides total optical control of the telescope has been further optimized and is now operated in a fully active optics closed-loop mode . This implies that the telescope functions automatically (much like a giant autofocus camera). During recent test observations, it continuously corrects the shape of the primary mirror and the positioning of the secondary mirror. The latter, a light-weight Beryllium mirror, is now providing so-called field stabilization at a rate of 10 times per second (10 Hz). This innovative system decisively improves the achievable image quality.

The site of the VLT Observatory, Cerro Paranal, was chosen after more than 8 years of careful testing by ESO, between 1983 and 1991. The meteorological conditions at this isolated spot in the Chilean Atacama desert were found to be among the best on the Earth for astronomical observations. This has been amply confirmed, both in terms of low atmospheric turbulence and high visibility in the infrared spectral region, because of the very low water content in the air over this extremely dry site.

The sharpest images yet

The stable atmosphere above Paranal, in combination with optimized, total control of the VLT UT1 optics, has now allowed some of the sharpest astronomical images to be obtained. In fact, during a period of particularly good conditions, images were recently taken with a quality even better than the excellent ones available at the moment of "First Light".

The present photo is a reproduction of a digital image that was obtained in the early morning of June 6, 1998. It shows the central part of the southern globular cluster Messier 55 (or NGC 6809) in the constellation Sagittarius. The exposure lasted 30 seconds and was made with the VLT Test Camera through an optical filtre isolating mostly red light (570 - 700 nm). It is here shown exactly as it was obtained ("raw image"), without any image processing . The brightness level in the reproduction has been set so that the fainter stars are well visible. The field shown is 83 x 83 arcsec; each pixel measures 0.0455 arcsec.

Images of a large number of stars over the entire field have been measured and consistently show a stunning image quality. The full-width-at-half-maximum (FWHM) is only 0.27 arcsec ! Moreover, the images are very nearly round - the measured image elongation is negligible at the 5% level (0.015 arcsec).

This capability used on earth would allow us to clearly see the two headlights of a car at a distance of no less than 1200 kilometres (the distance from Paranal to Santiago de Chile) - an astonishing performance of a ground-based telescope. It confirms that the VLT will be able to take full advantage of moments of particularly good observing conditions at Paranal and may soon begin to look beyond current horizons.

Present work

The UT1 has now entered the "Commissioning Phase" during which all systems are thoroughly tested and further tuned, leading up to "Science Verification" observations in the second half of August 1998. Thereafter, the two main astronomical instruments, FORS and ISAAC, will be mounted at the telescope and thoroughly tested. The UT1 will receive the first "visiting astronomers" for regular observing runs from April 1, 1999.

Further images of astronomical objects from the VLT UT1 will be published at irregular intervals.

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