A Century of Sky, Digitized

Deep within the photographic plate stacks at the Harvard-Smithsonian Center for Astrophysics, curator of astronomical photographs Thom Burns slips a delicate glass plate out of its brown paper sheath. He places it on a large, specialized scanner — custom-built for this purpose — preparing it for digitization, the astronomical markings encoded on its surface soon to be forever preserved in an online database. While there isn’t anything unique about this particular glass negative — its pale, filmy surface speckled with dark stars is a common sight in the stacks — there is nonetheless cause for trepidation.

This moment is the culmination of a project that has spanned decades: Digital Access to a Sky Century @ Harvard, or DASCH. Within the labyrinthian walls and filing cabinets of the Harvard College Observatory’s Plate Stacks reside more than 600,000 glass historical plates, spanning a century of astronomical research at Harvard. Members of DASCH have endeavored to digitize all directly imaged historical plates — which is no small feat, considering there are more than 400,000 of them. By digitizing such an expansive collection, the project has granted astronomers unprecedented access to a century of sky. With more than 1.2 petabytes of data now at their fingertips, astronomers unlock the potential for exciting discoveries.

As Burns sets the plate in place and flicks off the lights, the room falls silent. Soon, the machine begins its work — red lights flickering hypnotically as the scanner measures the position and brightness of each tiny region, then shifts the plate over with microscopic precision. The entire process usually takes about 85 seconds, according to the project’s operation lead Peter Williams, which means that when the project was underway, the team would complete on average around 200 to 400 plates a day.

This painstaking “labor of love,” as Williams puts it, finally comes to an end as the final plate is scanned. As the lights flick on to enthusiastic applause, the plate’s data is sent off to an online database, where it will eventually be made available on the DASCH website.

After the plate-scanning, the project’s founder, Jonathan “Josh” Grindlay of Harvard University, offers the entire team praise for the devotion they’ve shown to the project since 2005, when the first plates were scanned.

A Century of Fleeting Phenomena

DASCH provides astronomers with the benefit of hindsight, says Harvard astronomy professor Daniel Eisenstein: “We’re at a time now where astronomers are actively studying events that are arising now with very accurate new telescopes,” he explains. “But lacking that overall 100-year baseline can be a problem. So here, we can really unlock that and deliver it not just to scientists here, but around the world.”

In particular, astronomers are excited about the dataset’s ability to unearth new insights in areas of time-domain astrophysics, which includes the study of objects such as X-ray binaries and quasars that can evolve on human timescales.

However, the collection is more than just a repository of research data — the beginnings of photography and the progress of astronomical data collection can be traced through its images. “The collection represents the birth of photography itself,” Burns explains, from daguerreotypes to analog astrophotography. 

The collection boasts the first daguerreotype of the Moon, the discovery plate of the Horsehead Nebula, and the oldest photograph of a comet, among numerous other images of the Sun, Moon, eclipses, and stellar spectra.

The archive also traces the work of astronomers at the Harvard College Observatory, including that of more than 100 women who stewarded it from the 1880s through the 1950s.

DASCH preserves the meticulous records of 146 women, dubbed “computers,” who created, studied, and annotated the plates, including the likes of Annie Jump Cannon, Henrietta Swan Leavitt, Williamina Fleming, and Cecilia Payne-Gaposchkin.

Annotations include notes on spectral typing, on which Annie Jump Cannon built her spectral classification system, as well as plates marking the Cepheid variable stars that Henrietta Swan Leavitt studied to discover her famous period-luminosity relation. By digitizing the plates, “there’s so many more stories that we get to tell,” Burns says.

The Future: Project StarGlass

One project Burns still hopes to incorporate into DASCH is Project PHaEDRA, a “crowdsource transcription project” that records more than 2,500 logbooks from individual women computers. Burns hopes to link these logbooks to the corresponding digitized plates via their next big project: StarGlass.  “It's not only making the DASCH data more accessible to a wider group of people, but it is creating a system that we can continue on with digitizing,” Burns said.

Ben Sabath, a software engineer, has been developing the software to house StarGlass. “We’re still in the beta stage,” he explains as he clicks through the website, demonstrating how one can search for an object and sort by the different dates it was observed and by the astronomers who cataloged it.

The final plate in DASCH was scanned on Thursday, March 28th, 2024. Access the full DASCH archive here and see what StarGlass is all about here!