The discoverer of interstellar matter
At the Potsdam Astrophysical Observatory, Johannes Hartmann demonstrated the finely distributed gas between the stars in 1904 using the large refractor. He was the inventor of excellent instruments. The director of the observatory in Göttingen became head of the Argentine national observatory La Plata in 1921.
The astronomer Johannes Hartmann may not be known to laypeople, but is still well known to many professionals today because his name is inextricably linked to the Hartmann test method for telescope optics. It lives on in the Shack-Hartmann wavefront sensor for the adaptive optics of the world's largest telescopes. Less remembered are his contributions to astrophysics, such as the discovery of the interstellar medium at the Astrophysical Observatory in Potsdam, and his achievements as a hands-on observatory director in Göttingen and La Plata. Hartmann's life's scientific work was recognized in obituaries after his death in 1936.
An analysis of his extensive correspondence with the Heidelberg astronomer Max Wolf now allows a more complete overview of his career. In addition to the similar scientific interests of the two colleagues, who are almost the same age, their exchange also paints a picture of German astronomy and society at the beginning of the 20th century. In addition, Hartmann's grandson Michael Hartmann gave us an insight into his grandfather's estate.