[Previous] | [Session 3] | [Next]
D. Pascu, L. Winter, N. Zacharias, G. Wycoff (USNO)
The USNO 26-inch refractor has been used for photographic solar system astrometry for more than a quarter century. Two applications require a knowledge of the telescope's focal plane scale to better than 1 part in 20,000. The first is the astrometric intersatellite observations of the bright satellites of Jupiter and Saturn, which have maximum separations of about 1000 arcsec. The second is for the derivation of spherical equatorial (RA and Dec) positional observations of the major and minor planets relative to the Hipparcos and Tycho Catalogue stars.
We have located a calibration field in the Pleiades cluster suitable for the astrometric analysis of the focal plane of the 26-inch. On a 5x7 inch photographic plate (0.75 sq.deg.), the calibration field contains 10 Hipparcos stars as well as an equal number of Tycho stars. Several multiple-exposure plates of the field were taken in 1997, and measured with the newly automated StarScan measuring machine. A new value for the scale of the 26-inch will be presented, which is to be used with our 25-year observational set of plates of the Jovian and Saturnian systems. The focal plane aberrations of the 26-inch will also be discussed.