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Astronomy Project Produces Images

Photonics Spectra
Aug 1998
Richard Gaughan

The European Southern Observatory's Very Large Telescope delivered its first scientifically useful images May 25-26. Unit Telescope 1 is the first of four 8.2-m telescopes that will join up to three 1.8-m auxiliary telescopes to compose the 200-sq-m Very Large Telescope. When the telescope is operated in interferometric mode, the angular resolution will exceed even that of current space-based telescopes.

This image of a "strange" galaxy -- NGC 4650 in the constellation Centaurus -- is derived from seven individual exposures of the Very Large Telescope.

Maximizing resolution

Adaptive optics are used throughout the system to maximize the resolution. The surface figure of Unit 1's primary mirror is adjusted with 150 actuators controlled via wavefront sensors located at an image plane. To deform the surface and compensate for wavefront distortions, the mirror is relatively thin: just 175 mm. But just because it's lightweighted doesn't mean it's lightweight; the mirror blank consisted of 23 tons of Zerodur.

Unit 1's secondary mirror is more manageable: It's a mere 1-m-diameter lightweighted beryllium mirror weighing 51 kg. It is also actively controlled, moving at a rate of up to 10 Hz with five degrees of freedom to correct telescope aberrations and compensate for tracking errors.

Unit Telescope 1's primary mirror, shown during optical testing, was made with 23 tons of Zerodur but is just 175 mm thick.

Adaptive optics subsystems are placed at intermediate foci to provide more wavefront correction. As the technology advances, it will be possible to upgrade the subsystems easily, which designers believe will keep the Very Large Telescope at the forefront of astronomical observation for at least two decades after it becomes fully operational in 2003.

Full operational capability includes the ability of the telescope to function as an interferometer -- combining light from two to seven of its individual telescopes in perfect phase. More active optics, including adjustable delay lines, will allow measurement of differential delays as small as 5 nm. With that sensitivity, stellar motions can be quantified to identify stars with planetary systems.

Those types of measurements won't be possible for another five years, but Unit 1 already is taking advantage of the excellent viewing conditions at its 2600-m altitude in the Andes to begin significant observations. The science verification program scheduled for late August will attempt to identify the astronomical body responsible for gravitational lensing of distant quasars and correlate UV images with Hubble Space Telescope globular cluster pictures.

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