an astronomical instrument used to determine precisely the declinations of circumpolar stars in order to derive the constant of aberration and the constant of nutation.
A polar tube consists of a long-focus photographic telescope rigidly oriented toward the north celestial pole. The telescope continuously or intermittently photographs star trails for a period of several hours in the course of the apparent diurnal motion of the stars around the pole. In the resulting photograph the pole is at the center of concentric circular arcs described by the stars, and the position of the pole is determined by measurements. It is possible to calculate the constant of aberration from measurements obtained from a single year of observation and to calculate the constant of nutation from data of 19 years of observation. The precision of the results depends on the degree to which the orientation of the polar tube is kept fixed during the photographing in the course of the night.
Studies using a polar tube have been conducted at the Pulkovo Observatory in the USSR, as well as at observatories in the USA and Japan. The polar tube at the Pulkovo Observatory was installed in 1951. It has a focal length of 6 m, a 20-cm objective, and a field of view of 1• 50′.