Recording A Century of Night Skies Through A Scanner Darkly

CAMBRIDGE, Mass. — In the basement of a quaintly cramped building on the Harvard University campus, down a set of corkscrew stairs that would make a rollercoaster designer dizzy, the shelves and filing cabinets are spilling over with 100 years of stars. Glass photographic plates shipped from telescopes around the world document the Beehive Cluster as it appeared in 1890, or Cepheid variable stars as they looked in 1908. The glass plates — some 525,000 of them — serve as the only permanent record of the skies as seen by our forebears.

But the 170-ton database represents much more than an archive of astronomical history — it’s a potential gold mine for new discoveries, if only scientists could dig through it. With that goal in mind, a small collection of astronomers and archivists is using custom-built technology to bring this enormous data set into the digital age.

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When they’re finished in three or four years, the archive will consume 1.5 petabytes of storage. Any astronomer with web access can click on a star catalog and pull up an individual star’s light curve, showing how it has brightened or dimmed over time.

“We don’t have 100 years of data in any other form that is as complete as this,” says Josh Grindlay, a Harvard astronomer who leads the project, formally called Digital Access to a Sky Century @ Harvard. “There’s a real treasure trove of results here, and we’ve just been scratching the surface.”

As of Nov. 1, researchers had scanned 18,812 plates, a little more than three percent of the collection. But they are already bearing fruit, as Grindlay’s own research shows. He just submitted a paper to the Astrophysical Journal describing a potentially major finding: The mass-accretion formation of a Type Ia supernova. It’s a finding that brings Harvard’s astrophotography legacy to an almost touchingly complete full circle, if you’re into astronomical history. In 1912, Harvard astronomer Henrietta Swan Leavitt used photographic plates to discover Cepheid variable stars, the first “standard candles” used in cosmological measurements. A century later, another astronomer using the same images has re-discovered the second-generation cosmic yardstick, Type Ia supernovae, which just facilitated a Nobel Prize.

“It indicates that there is all sorts of incredible stuff lurking in 100 years of data,” Grindlay says.

Along with enabling new science, the project has been a lesson in how modern technology can preserve the past. Scanning humanity’s oldest sky photographs requires the most advanced imaging technology Harvard postdocs and engineers can assemble.

WHAT IS A PHOTOGRAPHIC PLATE?

To understand what the Harvard team is doing, it helps to have some background in astrophotography. Until the invention of the charge-coupled device (which won a Nobel Prize of its own), astronomers mostly used plate glass slides to take their pictures of the sky. The plates were commonly 8x10 inches or 14x17 inches, and coated on one side with a light-sensitive silver gel emulsion. Using a cloak to protect the emulsion, astronomers would slip the glass into a tailpiece on the end of the telescope and expose it to starlight. The exposures were later developed with mercury vapor or other chemicals, and then shipped back to Cambridge for further study.

Digitizing these plates is a lot like scanning photographic negatives — you just illuminate the negative and take a picture. Back in 2005, some engineers and volunteers from the Amateur Telescope Makers of Boston designed a high-speed scanner to do this. The digitized images would then be matched to star catalogs and used to extract light curve data for every star on every plate. The device had to scan one 14x17-inch plate or two 8x10-inch plates at a time, it had to be ultra-high resolution, it had to be kept cool and completely still to prevent blurring — and oh, it all had to fit through a couple tiny basement windows.

The digitizer, which was installed piece-by-piece, moves over the plates with linear servo motors and is guided with laser-etched markings. It sits on a granite slab which is itself protected by pneumatic legs to prevent any vibration interference coming from the antique building above it. An LED light source illuminates the plate emulsion in 8-microsecond bursts, and a CCD camera captures 60 overlapping images for each 8x10 plate. The final resolution is 11 microns per pixel, or 2,309 dpi — captured in 92 seconds. Put another way, in a minute and a half, the machine generates about the same amount of data as contained on a DVD for a typical hour-and-a-half movie.
In 90 seconds, Harvard's plate scanner generates about the same amount of data as contained on a 90-minute DVD.
The worst part? Before this process can start, the plates all have to be cleaned first and then loaded manually, a painstaking task that occupies much of Alison Doane’s time these days.

The plates are sort of a living library, and just like an old book, they’ve received their fair share of annotations during the past century. Astronomers would write on the glass side (not the emulsion side) to note what they were working on, Doane explains.

“There are very often handwritten notations, indicating a star, a brightness, a position, or circling a discovery,” she says. “We have to remove all of that, which is a big undertaking. It’s not something that we enjoy doing, but if we don’t do it, we’re going to see twice as many stars as are actually there on the plate.” After documenting the scribbles, she and a small staff of workers and students remove them by hand, using Windex and razor blades.

This will all get easier after Doane receives a custom-built automatic plate washer, which is being built with a grant from the National Science Foundation. It will work somewhat like a car wash undercarriage, driving a plate along a conveyor with the glass side facing down and the emulsion protected. The current prototype employs two types of brushes, but Doane says she and designer Bob Simcoe, who also oversaw the scanner development, are considering adding a razor too.

Grindlay hopes the washer, and an added staff member who started work in October, will help the project get up to full speed by January. He hopes to scan between 200 and 300 plates per day, ultimately ramping up to 400 per day. At that rate, they’ll be done in about four years.

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