BornNew York, New York, USA, 14 May 1909
DiedSanta Fe, New Mexico, USA, 31 October 1999
John Wainwright Evans was born in New York City on 14 May 1909. He died at home in Santa Fe, New Mexico, on 31 October 1999. He received a Bachelor’s degree in mathematics from Swarthmore College in 1932. From Harvard he received a Master’s degree in astronomy in 1936 and a Doctorate in astronomy in 1938. Bart Bok was his PhD thesis advisor. He was first director of the Sacramento Peak Observatory, which he built into a world-class solar observatory. From 1938 to 1942 he was an instructor and assistant professor of mathematics and astronomy at Mills College, Oakland, California, where he also worked at the Chabot Observatory. It was at this time that he independently invented the birefringent filter, which had already been invented by Bernard Lyot and Yngve Ohman.
Evans worked at the Institute of Optics of the University of Rochester from 1942 to 1946, where he developed a number of optical devices for military use. In 1946 he was hired by the High Altitude Observatory, working under Walter Orr Roberts until 1952, when he left to become director of Sacramento Peak Observatory. On 4 June 1946, 4 months after joining the staff of HAO, he was operating the Climax coronagraph by himself and spotted the “great granddaddy” of solar prominences in the eyepiece. He found he could not fit a prominence into the picture frame and rushed to ask Roberts for advice. Roberts replied that there never has been a prominence that large and that he must be doing something wrong. When Roberts came over to the observatory to check out the telescope, he reported that he nearly fainted at the size of the eruptive prominence, which appeared to be a huge magnetic torus gradually unwinding as it rose above the sun. That prominence became one of the signature observations of the Climax observatory.
While at the High Altitude Observatory, Evans developed a split-element birefringent filter, which is more transparent and has a wider field of view than the initial design. He developed an externally occulted coronagraph, which has been used on satellites such as the white-light coronagraphs on Skylab and Solar Maximum Mission. He also developed what is now known as the Evans sky photometer, which has been used to monitor sky brightness in site surveys around the world.
Soon after the end of the Second World War, Donald Menzel proposed that a second coronagraph station should be established in New Mexico as a supplement to the Climax coronagraph. As early as 1947 the shortcomings of the Climax site were becoming evident. There were too many cloudy days on the Continental Divide, increasingly due to jet condensation trails and activity at the adjacent Climax mine occasionally reduced the clarity of the atmosphere. In 1947 Evans sky photometers were used for site testing at Sacramento Peak, which had been chosen as a possible site because of its proximity to White Sands testing grounds, near Alamogordo. The Air Force was particularly interested in establishing an observing station on the summit of Sacramento Peak to track V2 rockets fired from White Sands. In 1949 observations at Sacramento Peak commenced with open-air telescopes run by Harry Ramsey and George Schnable. In 1952 a 26 ft spar was installed inside the new dome at Sacramento Peak, and Evans became director. He continued in that capacity until 1972 gathering an outstanding scientific staff and building one of the world’s leading solar observatories. Menzel had originally hoped there would also be a radio observatory at Sacramento Peak, but the extensive telemetry facilities at White Sands rendered it unsuitable. A solar radio observatory was established at Fort Davis, Texas, by Alan Maxwell in 1956, which became Menzel’s third western observatory, after Climax and Sacramento Peak. Although Menzel had hoped that these observatories would become part of an extensive program of solar physics at the Harvard College Observatory, Harvard was unwilling to provide tenured positions for Sacramento Peak astronomers. The Air Force stepped in and the observatory became the Air Force’s new Upper Air Research Observatory.
Evans served on the Editorial Board of Solar Physics from its inception in 1967 to 1976.
His research was characterized by constant innovations in the field of optics. His invention of the split-element version of the Lyot filter represented a major advance in the design of this type of optical filter. In 1949 he developed the design of the polarizing two-beam-interferometer form of Lyot filter that today is the basis of many helioseismic imaging programs. He made a further major contribution to the theory of narrow-band, tunable optical filters in his derivation of an exact analytic expression for the spectral transmittance of Šolc-type filters, which previously could be analyzed only by laborious numerical methods. He then applied the Šolc filter to a tunable monochromator, permitting the use of high-order gratings. He designed a double-pass spectrograph for reduction of instrumentally scattered light in solar absorption lines, which he incorporated into his design of the Sac Peak spectroheliograph which is still used in daily monitoring of solar activity. He pioneered the idea of using a sun-pointed solar spar on which multiple separate solar telescopes could be mounted, with a roller-type drive to give high precision and smooth guiding. He designed the solar flare patrol telescope, which became the standard for the International Geophysical Year. Evans also designed a Doppler–Zeeman Analyser for measuring solar magnetic fields with high linearity as well as one of the first vector magnetographs.
Evans led two eclipse expeditions to observe the height-resolved chromospheric spectrum to Khartoum in 1952 and to Puka-Puka in the South Pacific in 1958. For these he designed a slitless spectrograph and a jumping-film camera. Unfortunately, Evans had a terrible accident a few days before the Khartoum eclipse. He fell off the telescope, which somebody forgot to tie down, and it started to swing. He jumped off and hit the concrete pier, fracturing his ankle. Two of HAO staff, Robert Lee and Robert Cooper, succeeded in making the necessary optical alignments, guided by Evans lying nearby on a litter. The data from the Khartoum eclipse were critical in establishing the High Altitude Observatory as a major research institution in solar physics. In the hands of Richard Thomas and Grant Athay, the results of the Khartoum expedition firmly established the nature of non local thermodynamic equilibrium (non-LTE) in the chromosphere.
In 1958 he obtained data during the progress of a flare that showed, for the first time, associated changes in sunspot magnetic fields. In 1960, he initiated studies of small-scale motions in the solar atmosphere, obtaining the first quantitative measurements of the velocity amplitudes as functions of line strength and height in the solar atmosphere.
Evans was the third recipient (1982) of the Hale Prize of the Solar Physics Division of the American Astronomical Society. He won the David Richardson Medal (1987) of the Optical Society of America and was given Honorary Doctor of Science degrees by the University of New Mexico (1967) and Swarthmore College (1970). He received the Newcomb Cleveland Prize of the American Association for the Advancement of Science (1957) and was elected a Fellow of the American Academy of Arts and Sciences (1964). He received a number of prizes and awards from the Air Force such as the Rockefeller Award for Distinguished Public Service (1969), the Distinguished Civilian Service Award (1965), the Guenter Loeser Memorial Award (1967), and the Outstanding Achievement Award (1970).