Bigger, cheaper telescope starts active use

A telescope designed to give astronomers the advantages of one of the world's largest mirrors at a cost that isn't astronomical has finished its test phase and come into regular use.

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A telescope designed to give astronomers the advantages of one of the world's largest mirrors at a cost that isn't astronomical has finished its test phase and come into regular use. The Hobby-Eberly Telescope (HET) at McDonald Observatory (Mt. Fowlkes, TX) has a primary mirror 11 m edge to edge, with an effective viewing area of 9.2 m. It is, therefore, the third-largest optical telescope in the world, behind the two 10-m Keck telescopes on Mauna Kea, HI.

Already the instrument is producing results. The first scientific paper based on its observations is scheduled for the January issue of Publications of the Astronomical Society of the Pacific. Donald Schneider, an astronomer at Pennsylvania State University (Penn State; University Park, PA) and a member of the team that tested the HET during its commissioning phase, used the telescope to observe distant quasars and an extremely cool star. Edward Robinson, an astronomer at the University of Texas at Austin, has used it to observe an x-ray-emitting star in visible light.

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"The HET is a radically new design for a telescope and we're just trying to demonstrate that it works," Schneider said. "We're just thrilled with what we've got."

Moving tracking system in the Hobby-Eberly telescope follows an astronomical object across the sky, while the segmented mirror below remains fixed.

Getting telescopes from first light to their best performance takes a long time, he said. Various technical details have been worked on during commissioning, and the HET should realize its full potential within about a year. The telescope is now operating with the Marcario low-resolution spectrograph (see Laser Focus World, Sept. 1999, p. 40) and the upgraded fiberoptic Echelle spectrograph. A high-resolution spectrograph is to be installed early this year, followed by a medium-resolution device.

"What the Hobby-Eberly telescope does is allow us to build an $80 million telescope for about one-fifth the price," Schneider said. "Basically, it's just a much simpler design, [though] it's not as flexible as another telescope."

The HET, which cost approximately $15 million, was designed by Lawrence Ramsey and Daniel Weedman of Penn State (see Laser Focus World, July 1997, p. 75). The mirror consists of 91 hexagonal segments, each 1 m in diameter. The telescope is fixed at a 35° angle and rotates in azimuth only between observations. A moving tracking system follows the image of a moving object, such as a star, which is focused by the mirror.

The advantage of such a system is that the load on the assembly is fixed, and the total mass that moves is only about one-tenth of what it would be in a similarly sized telescope of conventional design, allowing the whole system to be built much less expensively. The trade-off is that only about 70% of the sky is accessible to the telescope, and the maximum observing time for a given object is roughly two hours before the object passes out of view. But given the choice between a $15 million telescope with some limitations and an $80 million instrument that might not get built at all, the compromise works to the advantage of university astronomers.

The HET saw first light in December 1996 and was dedicated in October 1997. A similar instrument is being planned for the South African Astronomical Observatory in Sutherland, South Africa.

Neil Savage

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