Laser-guided Star Facility demands beam point stability

July 11, 2001
The artificial-star reference system destined for the ESO's Very Large Telescope 34-in. YEPUN telescope at Chile's Paranal Observatory relies on a high-power beam delivery system developed by British laser-systems manufacturer Point Source.

The Laser-guided Star Facility (LGSF) currently being developed by the European Southern Observatory (Garching, Germany) for its Very Large Telescope (VLT) 34-in. YEPUN telescope at Chile's Paranal Observatory will use a high-power beam delivery system developed by British laser-systems manufacturer Point Source. When commissioned in 2003, the LGSF will enable observers to create a precise reference point in the sky on which to lock the telescope, the goal being to generate an artificial star high in the atmosphere in areas where there are no stars of sufficient brightness to provide reference. Beam point stability will be a critical requirement for project success.

According to Domenico Bonaccini of the ESO, extended analytical computations have confirmed the significant potential for the Laser-Guided Star at Paranal. While it offers a smaller stellar than a bright-centered natural star, the LGSF should considerably enlarge the accessible area of the sky. Bonaccini believes the development of the system is crucial for the Observatory's two near-infrared instruments�a NAOS/CONICA spectro imager and the SINFONI 3-DF spectrograph, both of which will ultimately be part of the Yepun telescope.

The artificial star system utilizes the strong Sodium presence in the atmosphere�s mesosphere 56 miles above the earth, an area with an absorption band at 532 nm (green). The system works by projecting a 532-nm beam from a Coherent Verdi diode-pumped solid-state laser into this area, where it re-emits at deep yellow, creating a bright spot where the absorption is high.

One issue that must be addressed by the LSGF is the need for precision optical alignement and hence, an absence of vibration or turbulence, as would occur if the laser was part of or close to the telescope�s optical assembly. Bonaccini reports that beam point stability is imperative, because any wandering of the beam would translate into reference inaccuracy.

The fiberoptics-based Point Source beam delivery system consists of a rugged, singlemode, polarization-preserving fiber protected by a tough, flexible, 0.2-in.-diameter stainless-steel outer jacket. This links directly to the laser via a Point-source-developed kinematic system. It also has prealigned and prefocused optical assemblies at the fiber ends. One benefit of fiberoptic beam delivery is the ability to mount the laser 82 feet away from the telescope and its ultra-sensitive Active Optics. This solution also eliminates the need for line-of-sight optics and complex opto-mechanical assemblies inside the telescope�s enclosure.

About the Paranal Observatory: The ESO-operated observatory is located 8530 feet above sea level in Chile�s Atacama desert. By 2003, the ESO plans to combine the four large independent telescopes with three dedicated 1.8-m auxiliary telescopes that will move on rails to form the VLT Interferometer to achieve unprecedented milli-arc-sec (0.001-in.) angular resolution for wavelengths from 300 nm to 25 �m.

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