By Emily Levesque, University of Washington
A leap forward in the studying and surveying of objects in the night sky came with the National Geographic Society-Palomar Observatory Sky Survey, which began in 1949. By then, astronomy had moved into the era of enormous mirrored telescopes, dedicated observatories on dark mountaintops, and top-quality glass photographic plates. Yet, its data was important.
Understanding Optics of Telescopes
George Ellery Hale’s immense 200-inch telescope began its operations in 1949, dwarfing any other telescope in the world and vastly expanding astronomers’ ability to see deep into the universe. One might expect that Palomar’s huge detailed survey of the Northern Sky would have been carried out with Palomar’s biggest telescope.
However, that wasn’t the case. Instead, the survey was done using the observatory’s relatively shrimpy 48-inch telescope. But, why?
Fields of View Are Important
To understand this, we need to understand the rules governing the optics of telescopes. The 200-inch telescope had an enormous parabolic mirror, dramatically magnifying a seemingly-small piece of the sky to capture the impossibly faint light from distant objects.
This meant that while the 200-inch was capable of achieving unprecedently detailed observations of the night sky, it could only do so on one tiny patch of sky at a time, thanks to its small field of view. It was a great telescope for depth and detail, but surveying the entire sky with it would have been like trying to cover the floor of a large house with postage stamps.
The 48-inch telescope, however, was specially designed to capture wide fields of view. Its mirror was spherical, not parabolic, and the telescope used a corrective lens to sharpen some of the known blurring that could occur at the edges of an image taken with a spherical mirror.
The mirror’s smaller size and unusual shape meant that, while it wasn’t as powerful as the 200-inch, it could capture much larger swaths of the night sky in a single observation. Now, instead of postage stamps, astronomers had huge square tiles at their disposal, literally.
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Using Photographic Plates
The 48-inch telescope’s survey data was stored on14-inch-square glass photographic plates, enormous for the time. Each plate stored a six-degree square view of the sky, equivalent in size to fitting 144 full moons onto one image.
These plates however were fiddly and difficult to work with. The telescope focused images to a curved plane rather than a flat surface, so astronomers would have to pre-bend the thin and brittle glass plates so that they would fit into the curved slot of the telescope’s camera. The plates also had to be carefully stored, prepared, and developed in darkness to avoid overexposing them and ruining a night of observations. However, the payoff was incredible.
The Palomar Observatory Sky Survey observed the entire sky visible from their mountaintop twice—once with photographic plates that were most sensitive to red light, and again with plates that were more sensitive in the blue. Combined, this meant that Palomar had assembled a complete view of the night sky in color. It sounds simple but knowing the color of an object can make a world of difference, allowing an astronomer to separate cold stars from hot stars, and young galaxies from old.
The Palomar Observatory Sky Survey
The Palomar Observatory Sky Survey took nearly 10 years and 2,000 plates, but when it was finished, it immediately became a treasure trove of references for astronomers. Now an observer who wanted to track down an unusual object they’d found during their work could simply reference a copy of the Palomar plates, using a coordinate grid and magnifying loupe to successfully identify and examine what they had been studying.
Astronomers at Palomar and elsewhere would scrutinize the plates taken from night to night and took great advantage of overlaps in the plates. From one night to the next, comparing two plates could alert them to a tiny moving object—maybe a new asteroid or passing comet—or the sudden appearance of a supernova.
Data from the National Geographic Society-Palomar Observatory Sky Survey is still used today. The data were eventually digitized, and an automated scanner generated a catalog that listed the tens of millions of objects contained on the plates. Still, by the end of the 20th century, a group of astronomers was ready to take the next step.
Time-domain Astronomy
Time-domain astronomy is studying how things change in the sky with time. Spotting an occasional odd variable star or happening to catch the brief appearance of a comet is one way to approach time-domain astronomy, but it depends a bit on luck.
Surveys are an excellent way to transform time-domain astronomy from lucky to methodical. Imagine a survey like Palomar, observing a vast array of stars not just once, but over and over and over again. Over time, these data come to represent not just a picture of the night sky, but a movie, tracking every change or variation as a function of time. A time-domain survey builds up an incredible store of light curves, monitoring stars that could be Cepheids, or strange erupting stars like the Boorong’s Eta Carinae. These stars could be orbited by other stars, or neutron stars, or even planets, and we’ll see signs of all of this in their light curves.
The 48-inch telescope at Palomar, once used for the observatory’s first great sky survey, has now been upgraded to operate robotically—complete with an automatic arm that can change the camera’s filters for collecting different kinds of data—and observe the night sky over and over. NASA’s TESS mission, short for the Transiting Exoplanet Survey Satellite, observes bright stars over and over again from orbit, as quickly as once every 30 seconds, to search for any changes in each star’s light curve, revealing everything from tiny outbursts to strange seismic activity inside the stars themselves.
Common Questions about the Palomar Observatory Sky Survey
The 48-inch telescope was specially designed to capture wide fields of view. Its mirror was spherical, and the telescope used a corrective lens to sharpen some of the known blurring that could occur at the edges of an image taken with a spherical mirror. The mirror’s smaller size and unusual shape meant that it could capture large swaths of the night sky in a single observation.
The Palomar Observatory Sky Survey took nearly 10 years.
Time-domain astronomy is studying how things change in the sky with time.
