The Photographic Zenith Tube (PZT) was designed and constructed to improve the accuracy of time-determination. It was also used at the Observatory for variation of latitude measurements. Operations with the PZT began in 1955 and ceased on 30 June 1984. The series includes three sets of drawings: blueprints of the U.S. Naval Observatory, Washington's Photographic Zenith Tube; a large set of design drawings for the RGO PZT by Sir Howard Grubb-Parsons and Co. Optical Works; and a set of miscellaneous RGO PZT drawings which include wiring diagrams, schematics and two floor plans of the PZT control room at Herstmonceux.