By Emily Levesque, University of Washington
Jim Gunn is something of a living legend to modern-day astronomers, a scientist who achieved the superhuman task of excelling in three different areas of research—theory, observations, and instrumentation. While most astronomers spend a lifetime building the skill and expertise to excel in just one of these fields, Jim Gunn has made pioneering contributions to all three.
Gunn’s Idea of a ‘Digital Telescope’
As a theorist—the same kind of work Einstein was famous for—Jim Gunn made groundbreaking strides in understanding the formation and evolution of galaxies. As an observer, mastering the art of capturing data from the night sky like Vera Rubin, Gunn gathered exquisite heaps of data on distant galaxies and entire clusters of galaxies, putting his own theory to the test in the same way that Eddington tested Einstein’s. Finally, as a brilliant instrumentalist, Gunn designed the telescopes and cameras that were used to study the cosmos.
In 1987, Gunn described an idea for a ‘digital telescope’, taking advantage of the larger and more efficient CCD detectors that were beginning to supersede photographic plates.
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Sloan Digital Sky Survey
Short for charge-coupled devices, CCDs contained silicon chips that were far more sensitive to light than glass plates. Moreover, they could translate the light they received into a quantifiable digital signal.
Used properly, an image from a CCD wouldn’t just show that a star or galaxy was bright or dim—it would spit out numbers measuring that brightness. This technology opened up a whole new world of precision comparisons and computational data in astronomy, and Jim Gunn recognized early on that CCDs could be an invaluable tool for a new and immense undertaking: the Sloan Digital Sky Survey.
The Sloan Digital Sky Survey has its own dedicated telescope in New Mexico, with a two-and-a-half-meter diameter mirror. The survey has observed almost continuously since the year 2000 and has built up a multi-color catalog of nearly one billion objects. More incredibly, the Sloan Digital Sky Survey has also captured spectroscopic data from more than four million objects.
Capturing Spectroscopic Data
Spectroscopic data is immensely powerful—Annie Jump Cannon used it to classify stars, and astronomers like Edwin Hubble used it to study the expansion of the universe. So, if it’s so much better, why don’t we simply acquire the spectra of everything in the night sky?
That question might sound absurd to many astronomers for one simple reason. Imaging in astronomy is fairly straightforward: you point a telescope at a patch of sky, take a picture, and then see what you’ve got. The telescope must be pointed pretty carefully, but if you’re a bit off to the left or right you can usually still see what you’re looking for. You can even take an image without knowing exactly what you’re taking an image of.
Spectroscopy poses a more daunting challenge. In spectroscopy, an astronomer must know exactly where their object of interest is, because they need to center on the object perfectly. The star or galaxy they’re studying needs to be lined up with a narrow slit or an optical fiber to ensure that this object, and only this object, is sending its light into the spectrograph. Get the object’s position wrong by even a tiny amount and you’ve missed the light completely; the spectrograph will instead be pointed an empty or impossibly faint patch of sky, and the data will come back blank or useless.
Incredible Digital Observing Capabilities
The Sloan survey solved this by using its own early images to design enormous observing plates, custom-designed to match specific patches of sky. Each plate was covered in tiny holes that would perfectly line up with the exact positions of hundreds of individual stars or galaxies. Each hole would, in turn, be plugged with an optical fiber that fed the light from the star or galaxy to a specific position on an enormous CCD.
With this design, the Sloan telescope could simply be loaded with a specific plate, plugged up with a forest of optical fibers, and pointed to the exact spot on the sky that corresponded to that plate. Then, in a single exposure, this one telescope would observe hundreds and hundreds of spectra that could be stored digitally and studied by any curious astronomer.
The Sloan Digital Sky Survey revolutionized the study of stars and galaxies, producing an incredible wealth of data. Like the Herschel’s catalog and the Palomar plates, astronomers still use Sloan data today. But unlike the Herschel catalog from several centuries ago, the Sloan database is still growing, as its telescope continues to pursue new and exciting scientific ideas that take advantage of its incredible digital observing capabilities.
Common Questions about Jim Gunn and the Development of the Sloan Digital Sky Survey
As a theorist, Jim Gunn made groundbreaking strides in understanding the formation and evolution of galaxies. As an observer, Gunn gathered exquisite heaps of data on distant galaxies and entire clusters of galaxies, putting his own theory to the test. Finally, as a brilliant instrumentalist, Gunn designed the telescopes and cameras that were used to study the cosmos.
CCD, short for charge-coupled devices, contained silicon chips that were far more sensitive to light than glass plates. Moreover, they could translate the light they received into a quantifiable digital signal. Used properly, an image from a CCD wouldn’t just show that a star or galaxy was bright or dim—it would spit out numbers measuring that brightness. This technology opened up a whole new world of precision comparisons and computational data in astronomy.
In spectroscopy, the challenge is that an astronomer must know exactly where their object of interest is, because they need to center on the object perfectly. The star or galaxy they’re studying needs to be lined up with a narrow slit or an optical fiber to ensure that this object, and only this object, is sending its light into the spectrograph.
