Thank you for the article “The Arecibo Observatory: Fifty astronomical years” by Daniel Altschuler and Chris Salter (Physics Today, November 2013, page 43). I’d like to add a historical note.

The first publication to discuss using a spherical reflector set in the ground and steered by a line source, as illustrated in the article’s figure 2, was published in May 1951 by engineers at the Air Force Cambridge Research Laboratories (AFCRL).1 The Department of Defense was interested in using what would become the world’s largest antenna to better understand the ionosphere for long-range radar and communications. Thus in 1959 officials from the AFCRL and Cornell University, home to Arecibo designer William Gordon, signed a contract to build the observatory. The project was funded by the Advanced Research Projects Agency, with Philipp Blacksmith of the AFCRL named as the project officer.

R. C.
C. J.
J. E.
Correction of Spherical Aberration by a Phased Line Source
Air Force Cambridge Research Laboratories
Cambridge, MA