The Photographic Zenith Tube (PZT) was designed by Dudley S. Perfect at the Abinger Magnetic Station in the early 1950s in order to improve the accuracy of time-determination. Its design was based on Airy’s reflex zenith tube and the recently completed Washington PZT. Made by Sir Howard Grubb Parsons & Co, it came into regular service at RGO Herstmonceux in 1956. It proved to be highly accurate and the best instrument of its kind in existence.

The PZT replaced the small transit instruments then in use as the primary method of astronomical time observation, and from 1956 to 1976 it was employed to determine Universal Time. It differs in operation from a transit instrument in that the eyepiece of the tube is fixed on the zenith. The passage of the clock stars are then photographically recorded, unlike the reflex zenith tube, and the clock times of each star passage are recorded. As a result, it is approximately five times more accurate than a transit circle and could also be used to measure latitude variation.

The instrument is extremely important as it provided the increasing accuracy that advanced clock design was demanding of the astronomical determination of time. The PZT’s excellent service (during which it only had one major overhaul) came to an end in 1976 when its purpose was taken over the Satellite Laser Ranger. The RGO PZT is currently in the collections of the Science Museum (Object Number 1990-758).