Solar observatory Einstein Tower: Data release of the digitized solar full‑disk photographic plate archive

We present solar full‑disk observations that were recorded at the Einstein Tower during the years 1943–1991 (solar cycles 18–22). High‑school students from Potsdam and Berlin digitized more than 3,500 full‑disk images during 2–3‑week internships at Leibniz‑Institut für Astrophysik Potsdam (AIP). The digital images cover a 15 cm × 15 cm region on photographic plates, which were scanned at a resolution of 7,086 × 7,086 pixels. The raw data are monochromatic 8‑bit images in the tagged image file format (TIFF). These images were calibrated and saved with improved photometric precision as 16‑bit images with 2,048 × 2,048 pixels in the Flexible Image Transport System (FITS) format, which contains extensive headers describing the full‑disk images and the observations. The various calibration steps include, for example, accurate measurements of the solar radius, determination of the limb‑darkening function, and establishing an accurate coordinate system. The contrast‑enhanced and limb‑darkening corrected images, as well as the raw data, are freely available to researchers and the general public in a publicly accessible repository. The data are published as a special data release of the Archives of Photographic PLates for Astronomical USE (APPLAUSE) project.