Development of a radio reference frame of very high accuracy has become necessary in order to obtain the required accuracy in spacecraft navigation for current and future planetary missions. The antennas of the NASA Deep Space Network are being utilized on a regular basis to obtain the positions of over 100 compact extragalactic radio sources distributed over the sky down to a declination of −45 degrees with uncertainties of less than 0.005 arcseconds.

The RATAN-600 radio telescope (Kaidanovsky & al. 1972) was designed for solving various problems of astrophysics and radiophysics (Pariisky & al. 1972, Gelfreikh & al. 1972). Because of its high resolving power and large collecting area, this instrument can be used to measure accurate positions of radio sources.

A catalog of 233 strong compact extragalactic radio sources which display optical counterparts has been prepared by the working group as a basis for the establishment of an almost inertial extragalactic reference frame. Optical properties of the catalog sources are summarized and further requirements for a refinement and extension of the present data are discussed.

Positions of the radio emission associated with stars were determined along with precise optical positions of the stars. The high precision obtained shows that radio star observations are a practical means of determining the relationship between the optical and radio reference frames.

A preliminary analysis has been made of data in the forthcoming edition of the Yale Parallax Catalogue to study the possibility of defining a system of observatory zero-points and external accuracies. Based on this analysis and those of several other authors, I conclude that it is not likely that a unique system of zero-points can be established. It may, however, be possible to establish some values for the average external accuracy of the parallaxes, but more work remains to be done on both of these issues. These conclusions are in agreement with those reached by several other authors.

The Lick proper motion program with respect to galaxies is summarized. The subject of extragalactic astrometry is introduced with estimates of proper motions for selected members of the Local Group of galaxies and more distant systems. The problems of measuring proper motions for the Magellanic Clouds is described in more detail.

The astrophysical problems that can be attacked via proper motions in star clusters are set forth and current techniques and opportunities are described. Cluster membership work continues to progress with recent work mixing plates from various telescopes, including large reflectors. More and better internal motion studies are needed to have a significant impact on the dynamical models. Absolute proper motions of clusters present a very difficult problem, but for globulars, at least, the data are desperately needed.

The first photographs for the Yale-San Juan Southern Proper Motion (SPM) program with respect to faint galaxies were taken with the collaboration of Columbia University in 1965. The first epoch photography was essentially completed in 1974 and plans are now under way to begin the second epoch observations in 1986.

In continuation of a long term programme of measurement of parallaxes and proper motions in the South Galactic Cap from plates taken with the UK Schmidt Telescope and measured in GALAXY at RGO, data for some 800 red stars brighter than B = 17.5 have been derived from two independent plate series. The parallaxes obtained from unfiltered IIaO plates show a large systematic error depending on magnitude compared with those obtained from IIaD plates with GG 495 filter. After correction, the combined trigonometric parallaxes show very satisfactory agreement with photometric parallaxes, and the average r.m.s. error of a combined parallax is found to be about ±0″.012 both from internal and external evidence.

The usefulness of Schmidt plates for measuring astrometric positions of extragalactic radio sources with an average uncertainty of less than 0.3 arcseconds is emphasized in the discussion of the results on 50 optical counterparts and by the comparison with corresponding positions obtained through radio interferometry.

This paper describes a short focus Astrometric Telescope (D=40cm; F=200cm) at the astronomical station of Campinas, in the State of Sao Paulo, which is a joint operation including the Federal University of Rio de Janeiro, the State University of Campinas, the Catholic University of Campinas and the Municipal Government of Campinas City.

The Lowell Observatory PDS is currently used in several projects which necessitate astrometric reduction of photographic plates. To meet this need, we have developed a series of computer programs for astrometric data reduction which enable the user to obtain maximum flexibility from the PDS while at the same time retaining ease of use. In its simplest form our astrometric “package” consists of four separate programs. The first two constitute the “data taking” part of the process and reside on a DEC PDP 11/34, the computer which controls the PDS. The second two programs handle “data reduction” and reside on Lowell's general purpose computer, a DEC PDP 11/55.

We have installed a dual-axis laser interferometer system on the Yale PDS 2020G microdensitometer to replace the linear encoders. Various diagnostic tests show that the laser interferometer is a superior encoder system for the PDS with a positional accuarcy of about 0.3 micron.

One of the promising ways for improving the precision of the angular coordinates (positions) of geostationary artificial satellites is the application of the long-focus astrograph, equiped with special equipment for tracking the satellites and recording the instants of exposure.

The present astrometric program of the Van Vleck Observatory began in 1967. Since then it has emphasized parallaxes and proper motions of stars of the lower main sequence which are not identified on the basis of proper motion and are therefore not biased towards high space velocity. Later an analysis was made from parallax and proper motion data from 70 stars in the spectral range dK3-M2 (Upgren 1973) which found the average external mean error in parallax to be 8.0 ± 1.7 mas (milliarcseconds). This external error was found to vary little from one parallax to another and to have no correlation with the formal internal errors for the individual parallax determinations. Hanson and Lutz (1983) confirm this result using the parallaxes of 14 members of the Hyades cluster also determined at Van Vleck. They find a mean external parallax error of 9.4 ± 1.8 mas and suggest that the proximity of the two determinations may be evidence that Van Vleck parallaxes may be characterized by a single external error. Their slightly larger figure may or may not reflect the relatively fewer plates and epochs of observation from which the Hyades parallaxes were determined.

A number of years ago the author (Stock, 1978) showed that astrometric positions free of magnitude- or color-dependent errors may be obtained from pairs of objective prism plates with opposite dispersion, since these allow monochromatic measurements as well as a restriction of the photographic density range without restricting the range of magnitudes that can be covered. Results from a series of plates taken originally for a radial velocity survey can now be compared with two new catalogues of positions from meridian circle positions, namely the Perth 70 (Høg 1976) and the Santiago 67 (Carrasco 1982) catalogues.

The Guide Star Selection System for the Space Telescope is being developed to find positions (± 0.25 arcsec) and magnitudes (± 0.4 mag) for guide star pairs for every possible ST target. The data available for the astrometric portion of this task are measurements (±1 μm) of plate surveys from the Palomar and UK schmidt telescopes and positional data from the AGK3 and the SAOC. Astrometric testing to date has concentrated on whole plate reductions in the area of the astrometric standard region in Praesepe. Described here are the test procedures derived to measure the effect of the astrometric accuracy of the guide star positions on ST pointing success. The most critical item in the pointing procedure is that the separations of the guide stars and the target over the ST field of view (28 arcmin diameter) be well-known. Any small zero-point shift or rotation of the field relative to the absolute system is tolerable.

Reflecting telescopes designed to do astrometry can obtain parallaxes essentially free of systematic errors. Improvements in detection and mensuration now make it possible to produce parallaxes of unprecedented accuracy without undue cost or effort.

The 1.56m astrometric telescope (Fig. 1) is designed mainly for determining trigonometric parallaxes of faint stars.

In order to keep the design as simple as possible, the telescope has only a Ritchey-Chretien focus and will accomodate plates 200 × 200 mm. We also plan to design several pieces of auxiliary equipment such as a medium dispersion grating spectrograph, a photometer, a reducing camera, and an electrographic camera, all of which are in the second stage of engineering. The whole project consists of four parts.

Quite a number of instances are known when Schmidt, mirror-lens system telescopes and generally reflectors are used for solving the problems of photographic astrometry.

This paper describes the use of the Ritchey-Cretien telescope (AZT-11) at the Abastumani Astrophysical Observatory for determining relative positions of the members of multiple systems.