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Article: Sloan Sweeps the Sky

The astronomers at Apache Point dislike clouds. On their 2,788-meter high, ponderosa pine-studded perch in the southernmost part of the Rockies, New Mexico’s Sacramento Mountains, a perfect night must be perfectly clear. Humidity must be low, with light winds, and neither lightning nor a full moon. John Barentine, one of the eight telescope observers at Apache Point, considers himself an amateur meteorologist as much as an astronomer.


The Sloan telescope points skyward at the Apache Point Observatory in the Sacramento Mountains, New Mexico.

Jason Lelchuk for AMNH

Such ideal conditions happen only one out of every three nights. That’s when the 2.5-meter telescope unfurls from its aluminum-slatted wind baffle to press on with the Sloan Digital Sky Survey. The project is a systematic scan of every object visiblethat is, visible to the telescopein one-half of the northern celestial hemisphere. This has never been done before.

Seven years of  observing at Apache Point has tallied over 100 million stars, galaxies, quasars, and other luminous space objectsmany unseen until now. “Huge amounts of data have come from this telescope,” says Bruce Gillespie, the observatory’s operations manager. “And when I say huge, I’m talking about more data than that contained by the digitized Library of Congress.”

Other sky surveys have been donemost notably by Caltech’s Palomar Observatory between 1950 and 1957, of about 50,000 space objectsbut never with the scope, technology, and usefulness of Sloan’s. The ultimate goal of the project, which is mostly philanthropically supported by the Alfred P. Sloan Foundation, is to create the first-ever map of one-quarter of the heavens in three dimensions. The task is now about 80 percent complete. When finished, the map will be consulted to help answer some of our thorniest questions about the structure and origins of the Universe. “It’s an issue of statistics and volume,” says Barentine. “A lot of the questions we have depend on that statistical informationbuilding up a picture of the Universe as a whole rather than just our particular little corner of it. The Sloan survey gives us access, for the first time, to the information that allows us to address the big-picture questions, which so far have only been the province of theorists.”

Spotlight vs. Floodlight

A portion of the Hubble Deep Field's renowned 1996 image of 1,500 galaxies in a narrow "keyhole" view of the Universe. The image covers an area of sky as wide as a dime viewed from 75 feet away and as deep as the visible horizon of the Universe.

STScI/the Hubble Deep Field Team/NASA

No conventional telescope, or even the Hubble Space Telescope, could do Sloan’s job. “The Hubble Deep Field has produced one of the most famous pictures in astronomy,” says Michael Turner, a theoretician at the University of Chicago and the Fermi National Accelerator Laboratory. “It is a deep and narrow image of the Universe containing about 1,500 galaxies. But the Hubble Deep Field is only one forty-millionth of the total sky. With Sloan, you're seeing one-quarter of the sky.” 

With its perpetually cloud-free vantage point in space, Hubble spotlights details about select space objects. Sloan’s much wider field of view casts a floodlight on the celestial landscape. As Earth’s rotation causes the night sky to roll by, Sloan stands at a fixed position on Apache Point, shooting a continuous strip of the heavens with its specially built 142-million-pixel camera. The strips are composited side by side like cosmological wallpaper, creating a seamless map of all the sky available to the telescope.

One sacrifice for Sloan’s wide field of view is its resolutionits images aren’t nearly as crisp as Hubble’s (click to compare). But its sensitivity is such that many very dim objects, both inside and outside our galaxy, are being registered for the first time. 

2D vs. 3D

Some of the panoply of luminous points in the night sky are bright because they are nearby stars. Others, such as quasars, are intrinsically bright, yet are billions of light-years from Earth. Sloan’s digital picture-taking registers the precise brightness of objects shiny and dim, and measures their positions relative to one another in two dimensions. Determining an object’s distance from Earththe added dimension of depthrequires an extra step  called spectroscopy.


Sloan observer John Barentine displays one of the survey's aluminum "plug plates." A plate is placed behind the telescope barrel to capture light from 640 selected galaxies in an area of space. The light is channeled to spectrographs via a fiber optic cable inserted in each hole.

Jason Lelchuk for AMNH

Spectrographs act as prisms by measuring a space object’s spectrum, or its light broken down into its constituent colors. “With spectra, you can measure very important physical parameters about the object, such as its distance, its temperature, chemical composition, and magnetic fields,” says Gillespie. “You do real physics with the data that from a spectrograph, rather than just take a picture of something.” Before Sloan, acquiring a single object’s spectra could take about an hour. Sloan’s spectrographs measure 640 spectra at once.

To save time, only a selected number of the 100 million space objects registered by Sloan get the extra spectrographic step: about 700,000 at current count. The distance data on the objects, acquired from the spectrum analysis, make up the points on the three-dimensional Universe map. A sample size of about a million objects is the minimum needed to get a sense of the overall distribution of the Universe. “Theorists want to understand the large-scale structure of the Universe, because our ideas about how the Universe began predict how galaxies are distributed in it today,” explains Turner. Turner and his colleagues at Fermilab are just one of the teams eager to test ideas about the early Universe against Sloan’s real-life, current-day data.

Then vs. Now

The Sloan survey team publicly releases its data in batches about 18 months after it acquires them. Using the data, astronomers have so far discovered the Universe’s most distant quasars, its dimmest stars and galaxies, a new class of dwarf stars, and even a previously unnoticed galaxy stuck right in our own Milky Way. “That aspect of the unknownof seeing things that no human being has ever seen before, of going to distances that no one has even really conceived of beforeis probably the most exciting aspect of this work,” says Barentine. The potential for going this distance has the scientists at Apache Point looking way beyond the clouds.

Related Links

Sloan Digital Sky Survey
Main page.

Sloan Digital Sky Survey: Sky Server
Data, visual tools, and other resources from the Sloan project.

Hubble Space Telescope

Hubble Deep Field

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