DASCH: Bringing 100+ Years of Photographic Data into the 21st Century and Beyond

TL;DR

This paper documents the DASCH project, which digitized Harvard's century-spanning photographic plate archive to enable modern, century-scale time-domain astronomy. It describes the hardware and software stack, including a custom plate scanner, astrometric/photometric calibration pipelines, and the daschlab data-access package, along with the resulting products such as FITS mosaics and extensive lightcurve catalogs. It reports the completion of the scanning effort in 2024 after overcoming floods, a pandemic, and storage failures, culminating in Data Release 7 with ~678 TiB of digital assets and a rebuilt data-access stack. The work preserves a valuable historical resource while enabling new analyses of long-term variability, transients, and activity across more than a century of observations, bridging archival heritage with contemporary astronomy.

Abstract

The Harvard College Observatory was the preeminent astronomical data center of the early 20th century: it gathered and archived an enormous collection of glass photographic plates that became, and remains, the largest in the world. For nearly twenty years DASCH (Digital Access to a Sky Century @ Harvard) actively digitized this library using a one-of-a kind plate scanner. In early 2024, after 470,000 scans, the DASCH project finished. Now, this unique analog dataset can be integrated into 21st-century, digital analyses. The key DASCH data products include ~200 TB of plate images, ~16 TB of calibrated light curves, and a variety of supporting metadata and calibration outputs. Virtually every part of the sky is covered by thousands of DASCH images with a time baseline spanning more than 100 years; most stars brighter than B ~ 15 have hundreds or thousands of detections. DASCH Data Release 7, issued in late 2024, represents the culmination of the DASCH scanning project.