130 years of spectroheliograms at Paris-Meudon observatories (1893–2023)

Abstract

Broad-band observations of the solar photosphere began in Meudon in 1875 under the auspices of Jules Janssen. For his part, Henri Deslandres initiated imaging spectroscopy in 1892 at Paris observatory. He invented, concurrently with George Hale in Kenwood (USA), but quite independently, the spectroheliograph designed for monochromatic imagery of the solar atmosphere. Deslandres developed two kinds of spectrographs: the ‘spectrohéliographe des formes’, that is, the narrow bandpass instrument to reveal chromospheric structures; and the ‘spectrohéliographe des vitesses’, that is, the section spectroheliograph to record line profiles of cross sections of the Sun. This second apparatus was intended to measure the Dopplershifts of dynamic features. Deslandres moved to Meudon in 1898 to build the large quadruple spectroheliograph. The service of Hα and CaII K systematic observations was organized by Lucien d’Azambuja and continues today. The digital technology was introduced in 2002. The collection is one of the longest available: it contains sporadic images from 1893 to 1907 (during the development phase) and systematic observations along 10 solar cycles since 1908. This paper summarizes 130 years of observations, instrumental research and technical advances.