Gerald Edward Kron ("Gerry") pursued high-precision photometry with photoelectric instrumentation of his design, primarily of variable stars and star clusters, aimed at advancing the field of stellar populations and interstellar reddening. He worked at Lick Observatory, at the Flagstaff Station of the US Naval Observatory, and at Mt. Stromlo.
Kron was born in Milwaukee on April 6, 1913, the son of a mechanic. He earned a graduate degree in mechanical engineering in 1934 at the University of Wisconsin, Madison, and was a protégé there of Joel Stebbins. He accompanied Stebbins and Albert Whitford to Mt. Wilson for summer observing runs beginning in 1935. He obtaining his PhD in astronomy in 1938 from the University of California, Berkeley, and worked at Lick Observatory as a staff member until 1965. During World War II Kron was a civilian scientist, first with MIT's Radiation Laboratory in radar development, then at Caltech in solid-propellant rocket development. He was Head of the Special Devices Group at the Naval Ordnance Test Station at China Lake, California. He married astronomer Katherine C. Gordon in Pasadena on April 22, 1946. He served as President of the Astronomical Society of the Pacific, and served two terms as President of IAU Commission No. 9 (Instrumentation). In 1965, when the astronomical staff at Mt. Hamilton moved to the new University of California campus at Santa Cruz, the Krons opted to move to Flagstaff, Arizona, where Gerald took the position of Director of the US Naval Observatory until 1973. He was a Senior Research Fellow at the Australian National University during 1974-1976. He continued his research program and was otherwise professionally active in retirement until the late 1980's. In 1985 he and Katherine moved to Honolulu, then returned to Arizona permanently in 1998. He died in Sedona, Arizona, on April 9, 2012.
His scientific work featured photometry of variable stars (eclipsing binaries, interacting binaries, pulsating stars, and eruptive variables) and stars of extreme luminosity (supergiants and late M dwarfs). His photometric measurements were of unprecedented precision and enabled such things as the determination of limb-darkening parameters for eclipsing binary systems. He pioneered near-infrared measurements of eclipsing binaries, and with Gordon used such data to argue for an extended electron-scattering atmosphere surrounding the Wolf-Rayet binary V 444 Cyg. He discovered the equivalent of sunspots on other stars. He and Gordon were the first to measure photoelectrically the light curve of a flare on a dMe star. He exploited the 6-color photometric system of Stebbins and Whitford to measure the colors of star clusters in the Magellanic Clouds and in the Fornax dwarf galaxy, thus discovering, with Ben Gascoigne, the population of blue globular clusters and the bimodality of the cluster color distribution. Based on his measured color-luminosity relation for stars with known parallax, Kron demonstrated how to determine accurate reddening values for more distant Cepheids and star clusters. Comparison of Cepheids in the Galaxy with those in the Magellanic Clouds showed that the latter were bluer and varied with lower amplitude. His work on reddening, the luminosities of Cepheids, and population characteristics of both Cepheids and clusters helped refine the distance scale.
His research covered a number of other topics, such as demonstrating that the near-infrared light of the night sky was dominated by OH (as opposed to N2) emission; measuring the brightness and color of the Sun on prevailing stellar photometric systems; with Nicholas Mayall, undertaking large-aperture photometry of Galactic globular clusters; and, with Donald Shane, determining the photometric depth of the Shane-Wirtanen galaxy counts and calibrating Fritz Zwicky’s magnitudes for bright galaxies.
Kron's engineering background and interests allowed him to make several important contributions to instrumentation. He and Whitford developed the first auto-guider for a telescope, as demonstrated on the Mt. Wilson 60-inch in 1936. Immediately following World War II, Kron was in large measure responsible for introducing photomultipliers, specifically the RCA 1P21, into routine astronomical use, based on his identification of low-noise tubes, amplifier design, and access to war-surplus recorders. He exploited André Lallemand's infrared-sensitive photocathodes and further developed photometers, for example optics for fields up to 15 arc minutes in diameter, and copper sulfate cells to block the red leak of ultraviolet filters. His designs were intended to be duplicated and were published in detail, one consequence being that amateur astronomers could take advantage of this technology.
With Mayall, in 1944 he met with Robert Sproul, President of the University of California, to discuss options for a large reflector for Lick Observatory, an idea that eventually led to the construction of the Shane 3-m Telescope. Kron was later involved in design specifications for the telescope and aspects of its construction.
In the late 1950's he began the development of an electronographic camera, whereby electrostatic optics re-imaged electrons produced by a large photocathode onto a nuclear-track emulsion. Similar technologies were being developed by J. D. McGee in London and Lallemand in Paris. The benefits with respect to photography included much higher quantum efficiency, a linear response to light, low noise, and a high dynamic range, features with enormous potential to advance astronomical two-dimensional photometry. A key problem with the McGee and Lallemand systems was either the need for a mica window to separate the plate from the vacuum, or the need to break the vacuum (and thus destroy the cathode) to retrieve the plate. Kron patented a valve internal to the evacuated volume that sealed off the cathode to enable the plate to be removed and replaced relatively conveniently, and without the need for a mica window. The Kron electronographic camera was adopted at a few observatories and even saw some commercial development. It enjoyed a run of scientific utility (e.g. surface brightness profiles of globular clusters and surface photometry of galaxies) before solid-state sensors made the bulky high-vacuum systems obsolete.
Gerald Kron led an active life and pursued a number of passions. As a graduate student he traveled from the Berkeley campus to Mt. Hamilton on his motorcycle, and later he enjoyed tinkering with British sports cars. He constructed model steam engines in his home workshop. He built an ultraviolet-sensitive photometer to monitor the amount of ambient UV light at various locations, altitudes, and times of day, to document human exposure levels. He once quipped that he "invented things that worked but nobody wanted, or that people wanted but didn't work." He recreated Daguerre's photographic process, obtaining a daguerrotype of Katherine with the family Willys Jeep. His interests turned to 16-mm filmmaking, documenting family trips to islands in the South Pacific. He had a lifelong interest in music and skillfully played the piano for relaxation. He is survived by his brother Robert and four of his five children.
Important sources consulted for this report:
oral history with David DeVorkin, 1978, American Institute of Physics history of physics
DeVorkin, D. 1985 Proc. IEEE, vol. 73, issue 7, page 1205, on the history of the development of photoelectric techniques in astronomy.
The professional papers of Gerald E. Kron are archived at the Lowell Observatory.