Mapping the Sky
Fermilab reviews the status of the Laboratory’s contributions to the Sloan Digital Sky Survey.
Attempts to map the sky are as old as the hills, but the one that presaged modern-day efforts was the Carte du Ciel.
Formally organized in 1887, the Carte du Ciel project was to involve 17 observatories scattered throughout the world, all with identical telescopes consisting of standard refractors with 13-inch-diameter lenses and 11-foot focal lengths. Astronomers of the day intended to produce both a small-scale photographic atlas of the sky and a catalog of stars, with their positions and brightnesses. Scheduled to be finished in five years, a completed Carte du Ciel never materialized.
Still, the Carte du Ciel was the first attempt to chart the universe on such a scale. And it set a precedent for later surveys, like the map of the northern hemisphere done by the Palomar Observatory in the 1950s.
Enter now the Sloan Digital Sky Survey, a collaborative effort of eight universities and research institutions that promises to set a new standard.
For, as speakers at a recent director’s review meeting at Fermilab emphasized, this new Survey will have no parallel anywhere in the world.
One quarter of the sky
When completed, the Survey will systematically chart one quadrant of the entire sky to an unprecedented level of resolution, producing a detailed map and determining the positions and absolute brightnesses of more than 100 million celestial objects.
The Survey will also measure the distances to a million of the nearest galaxies, providing a three-dimensional picture of the universe through a volume 100 times larger than any explored to date. The project will also record the distances to 100,000 quasars, among the most distant objects known, giving a unique look at the distribution of matter at the edge of the visible universe.
The review meeting, held on November 3, was a chance to brief not only members of Fermilab but also Jim Crocker, the Survey’s newly appointed associate director for program management. Crocker most recently served as program manager for the European Southern Observatory but perhaps is best known for proposing the method that succeeded in correcting the initial blurry vision of the Hubble Space Telescope.
The meeting was also the first time that Fermilab staff working on the project had assembled in one room for a major review. People all across the Laboratory are designing, constructing and assembling bits and pieces of the 2.5-meter telescope destined for Apache Point Observatory, in New Mexico.
Under Don Petravick, head of the High Performance and Parallel Computing Department, staff from the Computing Division have constructed the data acquisition system for processing the 10 to 20 terabytes of data the Survey is expected to accumulate. This system will take the photons of light from celestial objects after they have been converted into digitized signals, or pixels, and record them on magnetic tapes.
Under astronomers Steve Kent and Brian Yanny, of the Experimental Astrophysics Group, staff from Fermilab’s Computing Division and other institutions in the collaboration are developing the software for offline analysis. This software feeds the digitized information from the magnetic tapes into dataprocessing "pipelines," which will convert the pixels into a form astronomers can use to identify and measure the properties of stars, galaxies and quasars. For example, the monitor telescope pipeline, currently under the direction of astronomer Douglas Tucker, will analyze the data from the monitor telescope at Apache Point Observatory to produce a record of changes in the transparency of the atmosphere. Using these data, astronomers will be able to accurately calibrate an object’s brightness, as measured by the main telescope.
With overall coordination of task manager Bill Boroski, of the Technical Division, members of Fermilab’s Beams, Particle Physics and Technical divisions are creating several systems for use with the spectrograph, a device that will record the spectrum of light to analyze the distance, composition and age of each celestial object. One such system is the fiber mapper, associated with huge aluminum plates that will be inserted at the focal plane of the telescope. About the size of a large pizza pan, each plate is drilled with 640 holes corresponding to the positions of selected galaxies, quasars or stars in the sky. Into the holes, astronomers will fit optical fiber cables to record the light. The fiber mapper is the bookkeeper, letting the astronomer know which cable went into which hole, corresponding to which celestial object. Fermilab staff are also building the controls and interlocks that will drive the telescope’s motors, precisely controlling both the speed and direction of the telescope as required for each long sweep of the sky.
The project will hit several important milestones in the next several months. "We’ve reached a stage where, all of a sudden, people are bumping into each other," said Kent.
One milestone has just passed. Princeton University, one of the Survey’s collaborators, delivered to the observatory the imaging camera—the most complex camera ever built. In the next month, Fermilab staff will hook up the data acquisition system.
"We’ll find out if this thing really works," said Kent. Simulations were done, but "experience shows that simulations never test all of the possible features in the final system." Moreover, because the algorithms used to select objects for spectroscopy need to be the same from the beginning to the end of the Survey, the system has to work soon after the telescope is running. "Fixes can’t be made as you go along," Kent said.
In spring, the telescope should have what astronomers call "first light"—when the optics are installed and light passes through to make the first focused image. Soon thereafter, Fermilab’s controls and interlocks system for positioning and moving the telescope will be connected and the software tested and adjusted.
Excitement is growing, then, as instruments begin to assemble on the mountaintop in New Mexico to construct at last the long-awaited carte du ciel.
Now is when, Kent acknowledged, "morale goes up and, as the systems are put to the test of fire, panic sets in."