Theodore Walraven (1916-2008)

Dutch pioneer in optical instrumentation &
precision photometry
Theodore (Fjeda) Walraven was born in The Netherlands on 26 July 1916. He joined the Leiden Observatory after the war in 1946 and retired as a full professor in 1980. He earned his doctorate under the Dutch astronomer Anton Pannekoek with the thesis Line Spectrum of Delta Cephei, published in 1948, after which he contributed to a wide variety of observational astronomical projects and the building of astronomical instruments.
Walraven was a pioneer of high-precision photoelectric photometry and a genius in instrumentation. He made fundamental contributions to our understanding of variable stars. Together with the Belgian astronomer Paul Ledoux, he wrote a famous article on stellar variability in the Handbuch der Physik, Volume 51, pp. 353-604, published in 1958.
His observations started with remarkable studies using the recently built Zunderman 19″ reflector of multiperiodic variability of RR Lyrae. His subsequent description of the Blazhko effect was unsurpassed until only very recently.He invented simple methods to achieve continuous registration of a star’s brightness, which he later applied to his studies of SX Phoenicis and AI Velorum from the Leiden Southern Station near Johannesburg. During his whole life, he would remain fascinated by these stars and continued to improve the instruments he built to study them.
During the mid-1950s, he developed a photometer-polarimeter, with which he studied the polarization of the Crab Nebula. In collaboration with the Dutch astronomer Jan Oort, these studies revealed the importance of the synchrotron radiation in the Crab Nebula. The landmark paper by Oort and Walraven (B.A.N. 462, 1956) stands to this day as a classical example of well-conducted research.At the Leiden Southern Station, a wide variety of photometric programmes were executed, among which stands out the study on the southern classical Cepheids by Walraven, Muller and Oosterhoff. The large number of photometric studies resulted in the design of a 36 inch reflecting telescope, the so-called Lightcollector, which was erected in 1957. This telescope, built by Rademakers in Rotterdam, was fast, versatile and optimized for photoelectric photometry with small photometry diaphragms.
One of Walraven’s greatest achievements was a multichannel photometer, based on a polarization optics filter, which split the stellar spectrum into a set of regular bands that could be measured simultaneously. This yielded a five-channel photometric system of very high stability and efficiency that was particularly well suited for determination of the physical parameters of stellar photospheres.In 1978-79, the Walraven photometer and Lightcollector telescope were moved to ESO in Chile to start a new and very productive life on La Silla. After it had been in full operation for 32 years, the photometer was finally decommissioned in 1991. As a tribute to a great instrumentalist, it is exhibited in Museum Boerhaave, the Dutch National Museum for the History of Science and Medicine in Leiden.On 3 January 2008, Walraven died in Cornelia, a town near Pretoria in South Africa, where he lived since 1981.

MSc Specialisation at Leiden University, the Netherlands