Zenith Telescope, Photographic

an astronomical instrument for the simultaneous determination of latitude and clock corrections by observations of circumzenithal stars. It consists of a metallic column mounted vertically on a massive base. An objective measuring 20–25 cm in diameter and having a focal length of about 400 cm is placed in its upper portion. A basin of mercury is located beneath the objective at half the focal length. The rays from stars located close to the zenith pass through the objective and are reflected upward from the mercury surface to form point images of the stars several cm below the objective. A photographic plate holder is placed at this location perpendicular to the optical axis and is moved smoothly by a clock mechanism at right angles to the plane of the celestial meridian. The instrument is either controlled remotely or automatically according to a specified program. The precisions in the determinations of latitude and clock corrections (mean square error) from observations made in the course of one night are equal to +0.08 and +0.007 sec, respectively.

The first photographic zenith telescope was constructed by the American astronomer F. Ross and was set up in the International Latitude Observatory at Gaithersburg, Md. (USA) in 1911.