This is one of several diffraction gratings used on the the prime focus spectrograph of the 200-inch Hale Telescope at Palomar operated by the California Institute of Technology. The prime focus spectrograph, also called the nebular spectrograph, was designed and built at Caltech in the late 1940's for installation on the then-new Hale Telescope. Its original configuration used two prisms as dispersers, but in 1951 it was refitted for diffraction gratings, many of which were produced by the Babcocks in the Mount Wilson shops. The overall system was designed by Rudolph Minkowski and was used by him, as well as by Edwin Hubble, Allan Sandage, Maartin Schmidt and Jesse Greenstein, among many other prominent astronomers. The instrument remained in use continually from 1951 through 1973, providing a wealth of data on the redshift of distant galaxies, on white dwarf stars, and on the nature of radio galaxies, found to be optically stellar and hence called quasi-stellar radio sources, or quasars.This instrument assembly was donated to NASM by the California Institute of Technology in 1998. It is now on display in the Explore the Universe gallery. In the accesison process, the Museum conducted video interviews with two astronomers who had intimate knowledge of the device.