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February 3, 1997

Multi-object spectrograph helps keep Lick Observatory at forefront

By Robert Irion

A miniature forest of robotically controlled optical fibers has sprouted from the end of the 120-inch Shane Telescope at Lick Observatory near San Jose, letting astronomers capture and analyze faint rays of light from dozens of distant stars or galaxies at the same time.

Named the multi-object spectrograph (MOS), the device is the fruit of a collaboration between researchers at UCSC, which operates Lick Observatory, and Lawrence Livermore National Laboratory (LLNL). After more than a decade of design, manufacture, tests, and refinements, MOS became available for routine use last fall.

[Photo of Jean Brodie] Principal designer Jean Brodie, associate professor of astronomy and astrophysics at UCSC, says MOS has expanded the research horizon at Lick Observatory for three main reasons:

These attributes, says Brodie, make MOS unique in the world. "MOS is fast, accurate, and efficient," she says. "It offers all kinds of observational flexibility. This system doesn't try to compete with 10-meter class telescopes, but it is a perfect complement to them because it is optimized to perform entirely different kinds of science."

For instance, MOS is ideal for surveying the properties of large numbers of scattered objects, such as stars in swarms called globular clusters or groups of galaxies. It's not as efficient to use larger telescopes to study many such objects, says Brodie, because the telescopes cover tiny patches of sky--75 times smaller in the case of the W. M. Keck Telescopes. Further, a multi-object spectrograph dramatically cuts the prohibitive time it takes to observe numerous objects individually.

Brodie and her MOS collaborators plan to use the instrument to study clusters of galaxies that emit x-rays, stars in "open clusters," regions in the Milky Way where stars form, supernova remnants, and globular clusters around our sister galaxy, M31, in Andromeda. The latter study promises to shed light on the structure, evolution, and chemical history of this grand spiral galaxy.

Another UCSC astronomer, Puragra Guhathakurta, used MOS in September to probe distant galaxies that existed at a time when the universe was about two-thirds of its current age. Guhathakurta's team measured how quickly the galaxies rotate and examined how their brightnesses may have evolved over time. "MOS performed beautifully and exceeded our expectations in several cases," he says.

Astrophysicists Charles Hailey and William Craig, both now at Columbia University, are Brodie's primary collaborators on MOS. They oversaw an engineering team at LLNL that designed the fiber-optic positioning system and rebuilt an off-the-shelf robot to deploy the fibers.

The guts of MOS are a tribute to precision machining. Each fiber, about two-tenths of a millimeter wide, is housed in a three-millimeter button. The buttons each contain a prism the size of a grain of sand to direct light from the telescope into the top of the fiber. The buttons also hold tiny magnets to secure them to the surface of the instrument's metal plate.

To observe with MOS, astronomers prepare a list of precise coordinates on the sky for the objects they wish to study. A computer decides how to place the fibers most efficiently so that none of them tangle or collide. Then, a robotic arm darts into the optical spaghetti to rearrange it into the correct pattern. MOS currently has about 60 fibers, with another 40 on the way. Future plans call for upgrading the instrument to 250 fibers.

The fibers run down the side of the school-bus-sized telescope in a bundle some 130 feet long, then into a stationary spectrograph on the floor of the telescope dome. The spectrograph spreads the light rays into their component colors, or "spectra," for analysis. The Canada-France-Hawaii Telescope donated the spectrograph to Lick Observatory in the 1980s; a future spectrograph planned for MOS would further increase its efficiency by a factor of five.

Engineers, opticians, and technicians at the UCO/Lick Observatory shops at UCSC built the optical systems--designed by Professor Harland Epps--and electronics for MOS, as well as the "top end" structure that holds it at the telescope's prime focus. Other key members of the team that helped put the system in place at the 120-inch telescope are LLNL chief technician Steve Andrews and astronomer Ron Wurtz, UCO/Lick engineers David Cowley, Eric James, and Matt Radovan, Lick Observatory specialist Anthony Misch, current UCSC graduate student Kathleen Flint, and former UCSC graduate student Hank Donnelly.

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