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February 11, 2002

Advanced instrument destined for Keck Telescope completed at UCSC, begins shipment to Mauna Kea, Hawaii

By Tim Stephens

The most advanced optical spectrograph in the world began a two-week journey last Thursday from UCSC, where it was built, to the summit of Mauna Kea in Hawaii, where it will be installed at the W. M. Keck Observatory, home of the world's largest optical and infrared telescopes. The $10 million instrument will be a critically important tool for astronomers investigating fundamental questions about the origin of the universe.

Optician David Hilyard used polishing wizardry to create these lenses for a camera in the Deep Extragalactic Imaging Multi-Object Spectrograph, or DEIMOS, including the world's largest polished crystal of calcium fluoride (rear, on table). Nine lenses, smoothed to within millionths of an inch, will focus light from distant galaxies onto a detector. Photo: R. R. Jones
DEIMOS, shown here at the Lick Observatory Laboratories on the UCSC campus where it was built, will be mounted on the Keck II Telescope atop Mauna Kea, Hawaii. Photo: UC Observatories/Lick Observatory
DEIMOS (the Deep Extragalactic Imaging Multi-Object Spectrograph) will multiply by a factor of seven the power of the current Keck spectroscopy system. Observations of distant galaxies using DEIMOS will shed light on the formation and evolution of galaxies, the expansion of the universe, and the influence of "dark matter" on the large-scale structure of the universe.

Astronomers use spectrographs to separate the light from a distant object into a spectrum of different wavelengths, similar to the way a prism breaks up sunlight into a rainbow of colors. The result is a wavelength "fingerprint" of the object that can reveal a wealth of information about it. DEIMOS is a multi-object spectrograph, capable of gathering spectra from 130 galaxies in a single exposure.

"It's like having 130 Keck Telescopes taking spectra at the same time," said Sandra Faber, University Professor of astronomy and astrophysics. Faber led the team that designed and built DEIMOS at the Lick Observatory Laboratories at UCSC, starting in 1993.

"Multi-object spectroscopy is a relatively new technique that is just as important for astronomers as building bigger telescopes," Faber said. "It is technically very challenging, and we have had to push the state of the art in every respect to build this instrument."

The advanced optics in DEIMOS include a camera with the largest calcium fluoride lenses ever made. The fabrication and assembly of mechanical components had to be so precise that tolerances were held to less than a thousandth of an inch throughout the instrument. DEIMOS also has the largest spectroscopic detector in the world, gathering 67 million pixels of data per exposure (the equivalent of 300 handheld video cameras).

"The camera optics have set a new standard for the state of the art, and the detector package represents one of the finest of its kind in the world," said Joseph Miller, director of the UC Observatories/Lick Observatory (UCO/Lick).

DEIMOS will be mounted on the Keck II Telescope, one of the twin 10-meter telescopes at the Keck Observatory. When the spectrograph is in use, light gathered by the telescope passes through slits into the body of the spectrograph and is directed to a grating that spreads the light into a spectrum. The camera takes this dispersed light and focuses it to make an image of the spectrum from each slit. The detector measures the light intensity at each part of these spectra.

"The large detector is a huge asset to DEIMOS, because it allows us not only to gather many spectra at once, but also to spread out each spectrum to detect a broader range of wavelengths and therefore to see more details than would be possible with a smaller detector," Faber said.

The detector is a mosaic of eight of the largest charge-coupled devices (CCDs) ever made, each 2,000 by 4,000 pixels in size. From each exposure, the detector sends enormous quantities of data streaming through high-speed electronics to a computer for processing and storage. A typical night's work will fill up a 10-gigabyte hard drive with data, Faber said.

The 450-pound camera, designed by professor of astronomy and astrophysics Harland Epps, has nine lenses, including three made from calcium fluoride, used for its unique refractive properties. Unlike glass, calcium fluoride is a crystal that must be painstakingly grown in a vacuum furnace. The camera also has three glass lenses with "aspherical" surfaces, their curves deviating slightly from a precise spherical shape. It is extremely difficult to polish and test such a surface, and the Lick optical lab perfected a technique for making them using a special device to measure the shape of each surface to an accuracy of a millionth of an inch.

At nearly 20,000 pounds, DEIMOS is massive, requiring extraordinary measures to compensate for the effects of gravity on the instrument. An active flexure compensation system, never before used on a spectrograph, ensures that the images focused onto the detector are held steady to within a few microns (one micron is a thousandth of a millimeter, or 1/25,000 of an inch).

"It was a huge challenge to keep the perceived flexure--not the real motion of the materials, but as the detector would see it--to a minimum," said David Cowley, manager of the UCO/Lick Labs. "Without compensation the perceived flexure is less than 150 microns, and the active system reduces that to about 7 microns. It was a huge engineering feat to achieve this."

"This is the most ambitious instrument ever built at the Lick Laboratories, or anywhere else for that matter," added Miller. "It certainly ranks among the top instruments at any observatory in the world."

DEIMOS will be the prime instrument for a major survey of distant galaxies. The Deep Extragalactic Evolutionary Probe (DEEP) Survey will create the most comprehensive map of the distant universe ever attempted, probing tens of thousands of galaxies at the edge of the visible universe. The DEEP team includes Faber and several of her UCSC colleagues, as well as astronomers at UC Berkeley, the California Institute of Technology (Caltech), and the University of Hawaii. The researchers will look at galaxies billions of light-years away, in effect traveling back in time to study a critical phase in the formation and evolution of galaxies.

The DEEP Survey will focus on a period from about 5 to 8 billion years ago. In other words, it will look at galaxies whose light has taken 5 to 8 billion years to get here. The faint light from these galaxies poses formidable challenges, which DEIMOS is designed to overcome. By combining the light-gathering capacity of the Keck II Telescope with the exquisite sensitivity of DEIMOS, the researchers will be able to obtain detailed information from some 50,000 distant galaxies. Most importantly, they will be able to derive the mass of each galaxy, a crucial piece of information for testing theories about how galaxies formed and evolved through time.

The DEEP Survey will also gather information on the large-scale distribution of galaxies and their motions in space. "DEIMOS will map out in considerable detail how galaxies are participating in the expanding universe," Miller said.

A container ship is taking DEIMOS from the Port of Oakland to Hawaii, where it will be trucked to the top of Mauna Kea and lifted into the dome of the Keck II Telescope later this month. Installation and testing of the instrument is likely to take several months. "First light"--the first time astronomers will conduct observations with DEIMOS--is scheduled for June 2, 2002.

Most of the funding for DEIMOS was provided by the California Association for Research in Astronomy (CARA), a scientific partnership involving the University of California, Caltech, and NASA. CARA operates the Keck Observatory. The National Science Foundation also provided a major grant in support of DEIMOS.


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