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DASCH (Digital Access to a Sky Century @ Harvard)

The Harvard Astronomical Glass Plate Collection is an archive of roughly 500,000 images of the sky preserved on glass photographic plates, the way professional astronomers often captured images in the era before the dominance of digital technology. These plates are more than historical curiosities: they provide over a century’s worth of data that can be used by contemporary astronomers to trace how objects in the night sky change over periods from years to decades.

For that reason, researchers with the DASCH (Digital Access to a Sky Century @ Harvard) project are scanning the plates for digital storage and analysis. The process can also lead to new discoveries in old images, particularly of events that change over time, such as variable stars, novas, or black hole flares. The Harvard Astronomical Plate Collection is housed at the Harvard College Observatory (HCO), part of the Center for Astrophysics | Harvard & Smithsonian.

Curator Lindsay Smith Zrull analyzes one of the collection's glass plate negatives.

Image credit: CfA/Casey Atkins

CfA staff member Anne Callahan works to clean, scan, and categorize some of the 500,000 photographic glass plates in the collection. Each plate contains an astronomical image.

Image credit: Casey Atkins

Historical Data for Future Discoveries

Astronomical photography began almost as soon as photography was invented. HCO astronomers were pioneers in the field, using the large Harvard “Great Refractor” telescope to take photos of the Moon, stars, and other astronomical bodies in the mid-19th century. The Harvard Astronomical Plate Collection includes plates from more than 20 HCO-affiliated observatories located in the United States, Jamaica, Peru, South Africa, and New Zealand. The photos in the collection were taken between 1885 and 1992, and include a wide range of astronomical subjects.

DASCH researchers began scanning and digitizing plates in the early 2000s, and as of 2020 have archived over 400,000 plates. With so much data, DASCH offers astronomers the ability to compare regions of the sky as they appear today with their appearance at various intervals over the decades. For example, the DASCH project provided valuable data to the New Horizons spacecraft during its approach to Pluto.

DASCH also allows for identification of recurring phenomena, such as variable and binary stars, and one-time transient events, such as novas, giant flares, or the tidal destruction of stars by black holes. Over a century of newly digitized data also provides the possibility of discovering something entirely new, and leads to potential follow-up targets for current observatories.