Astronomical transit instrument
A telescope adapted to the observation of the passage, or transit, of an astronomical object across the meridian of the observer. The astronomical transit instrument is the classic instrument of positional astronomy, which is the study of the positions and motions of astronomical objects. The chief variants of the classic design include the vertical circle, the horizontal transit circle, the broken or prism transit, the photographic zenith tube and, most commonly, the meridian or transit circle.
The modern transit instrument has an objective (lens) with a diameter of 6–10 in. (15–25 cm) and a focal length of 72–90 in. (180–230 cm). The instrument consists of a telescope mounted on a single fixed horizontal axis of rotation. The horizontal axis has a central hollow cube (or sometimes a sphere) and two conical semiaxes ending in cylindrical pivots. The objective and imaging halves of the telescope are also fastened to the cube of the instrument, perpendicular to the horizontal axis. Rotation of the instrument in its bearings, or wyes, permits the optical axis to sweep only in the plane of the meridian. Because it is constrained to a single plane, it can be solidly mounted on massive piers, creating a stability not present in other telescope designs.
Meridian or transit circle
For many years, the major observatories of the world had astronomical transit instruments called meridian or transit circles. These instruments are similar to the transit instrument described above except that they are also capable of measuring the distance of the object along the meridian to obtain its declination, using a large, accurately calibrated circle attached to the horizontal axis. An accurate (atomic) clock is used as the scale for determining the right ascension of the object. See also Atomic clock.
Electronic detection systems, most commonly charge-coupled devices have improved the accuracy and efficiency of the observations, but they cannot be used far from the Equator due to the curved paths that the stars follow across the charge-coupled devices, and it is difficult to observe objects that are not point sources, such as the Sun and planets. See also Charge-coupled devices.
Applications
In 1997 the European Space Agency published the results of observations made with the Hipparcos satellite. The accuracy of the 100,000 star positions far exceeds anything obtainable with transit circles. In addition, modern astrographs with charge-coupled-device cameras can measure far more star positions than the transit circle and with greater accuracy. Nevertheless, transit circles still carry on specialized observing programs. The Carlsburg Meridian Telescope at La Palma in the Canary Islands is an example of a modern transit circle used to provide additional accurate star positions to improve the proper motions of previously measured stars and for the measurement of solar system bodies, especially asteroids (see illustration). It is completely automatic, being operated remotely via the Internet. Use of a charge-coupled device as the detector generally limits these instruments to areas near the Equator. See also Asteroid.
Carlsberg Meridian Circle, La Palma, Canary Islands. (University of Copenhagen)
