Vol. 628
In section 9. The Sun

Reconstructing solar magnetic fields from historical observations. V. Sunspot magnetic field measurements at Mount Wilson Observatory

by A.A. Pevtsov, K.A. Tlatova, A.A. Pevtsov, et al., 2019, A&A, 628, A103 alt

There is a treasure trove of well over a century of solar observations sitting in the archives of many observatories, consisting mainly of drawings from the pre-photographic era. Many of these data are unique and record the Sun in states that are valuable for understanding the dynamo and even climate change. This paper is an exemplar study of how to recover that wealth of information and it also presents a complete study of the visual sunspot observations from the Snow 150 ft telescope at Mount Wilson, spanning the 20th century. Sunspot distributions, structure, and even the solar radius are recoverable. Additionally, the authors present a particularly valuable guide of how the observations were conducted and quantified, and they explain the procedure necessary for recovering the information using modern digitization methods and cross-calibration with other simultaneous Zeeman magnetic field measurements.