This eyepiece assembly was designed to allow an astronomical observer to view the field of view of the 200-inch Hale telescope at its prime focus. The eyepiece was focused on the optically polished flat surface of the entrance slit of the Prime Focus Camera showing the observer the field of view, minus that thin portion of the field that had actually passed into the spectrograph through the slit opening. Therefore, during an exposure, which could take hours, the observer could carefully move the image of the galaxy up and down the slit to evenly expose its spectrum inside the camera. This produced a "widened" spectrum allowing for the absorption and emission lines to be distinct and easily measurable.

Called either the prime focus spectrograph or the nebular spectrograph for the 200-inch telescope, the assembly this eyepiece was part of was designed by Rudolph Minkowski and built at the California Institute of Technology in the late 1940s. The instrument remained in use 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. In the accesison process, the Museum conducted video interviews with two astronomers who had intimate knowledge of the device.