In memoriam Ulrich J. Schwarz (1932-2018)

On 17 October 2018, Ulrich J. Schwarz died peacefully, after a long illness. Ulrich was born in Bern, in Switzerland, on 11 October 1932. He thoroughly learned mathematical statistics from his father at age 17. He studied mathematics, physics and astronomy at the University of Bern, worked with Geiss and Oeschger on cosmic radiation, and on isotopic abundances in meteorites and on Earth, and obtained his doctorate in February 1962.

In 1957/58 he worked with Charles L. Seeger on radio astronomy receivers, at Leiden Observatory. On 9 December 1961 he married Elisabeth van Woerden, whom he had met at musical evenings in the Observatory assistants' home, organized by Kwee Kiem King. He joined the Kapteyn Institute at Groningen in January 1962, as a staff member for radio astronomy. He served on the staff at Groningen until he retired in 1995.

Between August 1962 and August 1964, while Hugo van Woerden was on leave of absence, Schwarz led the growing radio astronomy group. In the following years, he used the Dwingeloo Radio Telescope for a variety of projects, especially on the properties and space distribution of HI clouds. In 1968-70, he worked at the Division of Radiophysics, CSIRO, in Sydney (Australia), with Radhakrishnan and others, and obtained experience in radio interferometry.

Following the inauguration of the Westerbork Synthesis Radio Telescope (WSRT) in 1970, Schwarz concentrated on the use of WSRT, in close collaboration with both local astronomers (Paul Wesselius, Jacqueline van Gorkom, Jaap Bregman) and many excellent scientists attracted from other countries (Ron Allen, Ron Ekers, Miller Goss, Peter Shaver, Kurt Weiler, Linda Sparke and others). Prominent research projects included small-scale interstellar structure, supernova remnants, structure and motions of gas around the Galactic Centre, HI in SO galaxies and ringed galaxies, high-velocity clouds. His work on high-velocity clouds (HVCs), together with Bart Wakker, Reynier Peletier, Peter Kalberla and Hugo van Woerden, led to the first reliable distance determination for an HVC, and to a co-editorship of the first book on HVCs.

Of great importance was his work on software, notably on the mathematics of CLEAN: the correction of measured brightness distributions for the effects of incomplete coverage of the aperture plane. His software was called “clever" (Jacqueline van Gorkom) and "mathematically precise and reliable" (Martin Vogelaar). He gave excellent courses on radio interferometry, and became the key person in the use of the on-campus two-element radio interferometer PQRS first proposed by Allen and Ekers.

After his retirement in 1995 he moved to Arnhem, and developed radio astronomy at the Radboud University in Nijmegen. A two-element interferometer was constructed there, used by students, and on his retirement in 2012 it was named the Ulrich J. Schwarz Radio Interferometer.

Since 1970, until his last days, Ulrich has been deeply concerned about the problems of the environment.

Ulrich was a kind and modest person, whose research projects were strongly based on friendly collaborations. He played the flute in family concerts. He was an accomplished painter, and left a large heritage of water colours and woodcuts, including many Observatory paintings. He will be sadly missed by many friends and colleagues.