Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984coas.conf...45m&link_type=abstract
In JPL Cometary Astrometry p 45-47 (SEE N85-25015 14-89)
Astronomy and Astrophysics
Astronomy
Astrometry, Astronomical Photography, Comets, Errors, Photographic Plates, Astigmatism, Diaphragms (Mechanics), Mirrors, Reference Stars, Stellar Magnitude
Scientific paper
Coma, astigmatism, and great differences in stellar magnitudes between photographed object and reference stars constitute the main sources of errors in measuring positional plates. These three sources of error can easily be eliminated by the method used at the Klet Observatory for obtaining precise observations of faint objects. The astrometric plates are taken by the method of two diaphragms. The first diaphragm, with a small central aperture; is located in front of the photographic plate. The second diaphragm is situated in front of the mirror. By a very short (of the order of tens of seconds) exposure a sufficient number of reference stars can be obtained throughout the entire plate. The stars are very well defined to the very edge of the plate and are easy to measure. Moreover, this method makes it possible to use plates of larger dimensions than usual so that it is always possible to find the necessary reference stars.
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