The Photographic Zenith Tube is located on the grounds of the U.S. Naval Observatory (USNO), at Number One Observatory Circle in Washington, D.C. The early 19th century buildings house the USNO, one of the worldwide national time standard observatories. At the time of the PZT installation on these grounds by Larry Averil of Boller and Chivens, it was of the latest state-of-the-art in establishing exact time through celestial observations.
The USNO shares these grounds with the U.S. Vice President’s mansion: www.whitehouse.gov/about/vp-residence

Larry Burris of Boller and Chivens pictured on right.

This structure houses the Photographic Zenith Tube, used by the US Naval Observatory to establish absolute time. It was designed by John O’Rourke of Boller and Chivens.
It is a basic telescope with a four element 26 inch diameter lens cell at the top of a 15-foot high vertical structure.

The roof on the structure opens to let starlight in. The starlight passes through the 26-inch lens cell and projects down to a 18-inch diameter mercury pool mirror at the bottom of the structure.                                                          The star image is then reflected back to the focal plane just under the 26-inch lens cell assembly.

A glass photo plate at the focal plane records the star images and a time reference is recorded at the time the image took place. The lens and photo plate are then rotated 180 degrees and the same star images are exposed on the same plate. The time is recorded again when the second image took place.

Calculations averaging both positions of the star and the averaging the time difference will give an exact celestial time which can verify and/or correct the master Naval Observatory published exact standard time.
This whole process  became obsolete with the advent of atomic clocks.

John James of Boller and Chivens  working on the Zenith Tube housing

Dick Nason of Boller and Chivens working on the rotatable Camera Lens Assembly

Pat Patton of Boller and Chivens assembling Camera Section