Zenith telescope
Center scopes are high-precision measuring instruments used in astronomy and geodetic earth measurement . They are a special further development of the meridian circle and are used for the precise determination of star locations and the direction of the perpendicular . In the vicinity of the zenith , such measurements are best because of the low level of atmospheric disturbance.
In addition to the star coordinates, important measurement results are the astronomical latitude and longitude or the local sidereal time . In cooperation with other observatories, the polar movement of the earth and a uniform, international system of length and time can be established.
Instrument technology and research goals
Zenittelcopes are lens telescopes with a focal length of a few meters and an opening width of 20 cm to 30 cm , which are mounted in the vertical plane of the local meridian . With special methods and micrometers (see Horrebow-Talcott method ), differences in the zenith distance of star pairs and / or the times of their meridian passage are measured. You can do that
- calculate the exact positions of stars near the zenith,
- what enables the establishment of the " fundamental system " of celestial coordinates in the global network of some such observatories (similar methods are also used by scanning astrometric satellites ), or
- Derive fluctuations in the direction of the perpendicular - see polar movement and plate tectonics of the earth
- monitor the rotation of the earth and model its slowdown (1 to 2 milliseconds per century).
The restriction to stars close to the zenith is a hindrance for large-scale measurements and analyzes, but increases the accuracy quite significantly - namely to about 0.01 "(30 cm on an earth radius of 6357 to 6378 km).
Specifically, the zenith is chosen for such precision measurements because
- the atmosphere there has the least influence on the telescope image of the stars,
- the telescope bending and secondary axis errors are largely compensated,
- the same pairs of stars can be observed periodically (up to a few thousand depending on the telescope type)
- and therefore certain systematic errors can be eliminated.
Zenith telescope in the 17th and 18th centuries
The first ideas for a zenith telescope go back to the 17th century. Robert Hooke had built a prototype, which he called the “Archimedean Engine”, into Gresham College to demonstrate the heliocentric world system. Using the star Etamin in the kite, which almost goes through the zenith of London, he wanted (in vain ) to measure its assumed annual change in position ( parallax ).
After the fire in London in 1666, Hooke and Christopher Wren used the Doric column erected as a memorial to the fire for this experiment and attached a lens to its top. But it wasn't stable enough against wind. 50 years later, James Bradley and Samuel Molyneux repeated the experiment with a much smaller, but more stable mounted zenith telescope. Instead of the (very small) parallax, they discovered the aberration of starlight (20 "), which was the first time in 1728 that the motion of the earth could be detected.
Large zenith telescopes
Reflector telescopes with a liquid primary mirror can only be constructed as zenith telescopes. The Canadian Large Zenith Telescope has an opening of six meters. Its mercury level is kept in its parabolic shape by rotation.
The Arecibo radio telescope with a diameter of over 300 m qualifies as the largest zenith telescope .
literature
- Karl Ramsayer 1969: Geodetic Astronomy . Handbook of Surveying Volume IIa, JB Metzler-Verlag Stuttgart, 902 p. (p. 243-247 et al.)
- G. Damljanovic, G. Gerstbach, M. de Biasi, N. Pejovic (2003): CCD Technique for Longitude / Latitude Astronomy. Proceedings XIII Nat Conference p. 229-235, Astr. Ob. Belgrade No.75, 2003