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By R. C. Stone, C. C. Dahn (auth.), Erik Høg, P. Kenneth Seidelmann (eds.)

Astrometry is at the threshold of serious adjustments considering the fact that this decade, by myself, is witnessing an development of stellar positions reminiscent of the whole development of the former centuries. The Hipparcos satellite tv for pc has concluded its observations, and the catalog is in training. initial effects guarantee that the Hipparcos catalog will supply positions, parallaxes and annual right motions for over 100,000 stars with accuracies of 1.5 milliarcseconds. additionally, the Tycho catalog will offer positions of approximately 30 milliarcseconds accuracy for over 1 million stars, and annual right motions with three milliarcsecond accuracy will for this reason be ob­ tained by way of first epoch positions from the Astrographic Catalog. Optical interferometers at the flooring are starting operation, and those tools supplies observational accuracies of roughly one milliarcsecond. additionally, the conventional reference body in accordance with the joys­ damental Catalog of brilliant stars is being changed by way of the extragalactic ref­ erence body, in response to radio resources with accuracies of 1 milliarcsecond. therefore, astrometry will switch from a basic reference body outlined when it comes to the dynamical reference body of the sunlight process with accuracies of a hundred milliarcseconds to a space-fixed, extragalactic reference body with accuracies of 1 milliarcsecond. destiny astrometric observations might be within the 1 -100 milliarcsecond accuracy diversity. there are various techniques for destiny astrometric tools in area. almost all these provides sub-milliarcsecond astrometric accuracies.

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Additional info for Astronomical and Astrophysical Objectives of Sub-Milliarcsecond Optical Astrometry: Proceedings of the 166th Symposium of the International Astronomical Union, Held in the Hague, The Netherlands, August 15–19, 1994

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As expected, observations with the Mark III Interferometer indicate that most of the FK5 stars in this group have positions within their estimated error ranges (about 50 mas in both coordinates in 1988 and 1989), and some stars do have position errors as large as 100 mas at present. Table 4. 6 Offset in Dec!. 616 2 -4 -11 34 -14 -3 -3 -3 -9 -9 XIAOPEI PAN ET AL. 18 Offset in Decl. for FKS Stars 160 + 88 089 140 120 .. s2O 0 3l-2O i o • :j:: -40 0_60 • •• .........................................

Quasi-absolute catalogs are not necessarily derived from transit circle observations, and in fact some ofthe best, most notably HIPPARCOS and the astrolabe catalogs, are derived from other instrumentation. 3. DIFFERENTIAL CATALOGS Catalogs that have been compiled using the star positions of another catalog as reference stars are termed differential. Each night's observations are generally a mix of program stars and reference stars, and the reductions are made relative to the reference catalog. These comments apply to observations made over both wide angles by transit circles and narrow fields by astrographs.

6 ms (= 9 mas) in RA. for a group of stars covering a range of 90° in RA. and 45° in Decl. Such high precision comes from the good thermal control of the instrument, monitoring of variations of the delay offset, and the two color data analysis technique. Repeated observation were made in 1989 and had the same average formal 10" errors as in 1988. 5, suggesting average external errors of approximately 9 mas in Decl. A. for each year. As expected, observations with the Mark III Interferometer indicate that most of the FK5 stars in this group have positions within their estimated error ranges (about 50 mas in both coordinates in 1988 and 1989), and some stars do have position errors as large as 100 mas at present.

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