The realization of an optimum astronomical reference frame depends on well-defined connection between the systems of optical and radio positions of objects. For this the physics of radio objects are to be studied carefully. Besides this it is necessary to investigate how to get accurate positions of the optical counterparts of these objects as benchmarks for establishing a radio-optical reference frame. As the optical sources are very faint, photographic telescopes with robust light-gathering power are to be used.

For 15 years research work has been done using plates of the 134/200/400 Tautenburg Schmidt telescope for determinations of photographic positions of stars and extragalactic objects. An overview of some special results obtained at Dresden Lohrmann Observatory and Potsdam Central Institute of Astrophysics of the GDR Academy of Sciences will be given.

The measurements of AGK3 stars carried out with the automated measuring machine MAMA in Paris were used for the estimation of the accuracy of stellar positions on Tautenburg and CERGA plates. The results show good coincidence of stellar positions derived with both Schmidt telescopes. The achieved accuracy is high enough to use combined observations for improving present positions in the catalog AGK3.

From measurements of Tautenburg Schmidt plates with the APM in Cambridge positional accuracies per plate of 0.″05 for stars and of 0.″10 for galaxies were achieved. With 0.″3/100a accuracy in a single stellar proper motion we obtained the absolute proper motion of the M3 globular cluster in good agreement between the two pairs of plates used.

Optical positions of objects from the IAU Commission 24 Working Group list of benchmark radio sources have been derived which will contribute to the link of the radio and optical reference frames. Results for 11 objects with an r.m.s. position error of 0.″2 are presented and discussed.

In April 1985, we started a program to measure trigonometric parallaxes for faint southern high proper motion stars, using a CCD at the Cassegrain focus of the Cerro Tololo Interamerican Observatory (CTIO) 1.5-m telescope. The program stars (mR > 16; μ ≥ 1 arcsec/year) were selected from the LHS Catalogue and the University of Chile proper motion program.

The X and Y positions of the stellar image centroids were obtained using the algorithms of DAOPHOT program packages. The precision of the measurement of one stellar image is about 6 milliarcseconds (0.02 pix). For stars with 16 < mR < 19.5 a precision of 2 milliarcseconds in the parallax determination can be obtained in a one year period. Trigonometric parallaxes for some stars common to other parallax programs are given, showing an agreement with those results within the quoted mean errors.

Several technical aspects of the present program are also discussed.

A special series of observations of the groups of stars performed in Pulkovo, Chile, Kharkov and Belgrade were used to compare the mean errors of the right ascension by means of the method eliminating the influence of the errors of the source catalogue. The results show the advantage of the small transit instruments over transit circles of the classical type.

The Time Service Catalogue (TSC), which is obligatory for use in all USSR Time Services, was created for the epoch 1958 in the 1970s. Research done in the succeeding years detected a seasonal variation of TSC errors. Thus, a new Time Service Catalogue (TSC2) was required, in order to remove the seasonal errors as well as to satisfy the increasing demands for accuracy in the determination of universal time. During 1986-88 the data of Kharkov Common Time Service for 1980-1987 were reanalyzed using the new system of astronomical constants and two new catalogues were produced.

A major international effort to observe and compile the results of observations from many transit circle programs into a single catalog of positions referred to the FK4 system came to a conclusion with the completion and distribution of the Southern Reference Star (SRS) catalog of 20,488 stars. Previous discussions focussed on the adjustments to the observational material to refer it to the FK4 system and on the random errors as estimated from residual differences. In the present discussion, we give the results of internal comparisons which have been made between the individual contributing catalogs and the final combined SRS catalog. Also, results of a comparison between the SRS catalog and the AGK3R catalog are given where they overlap in the declination zone from +5 to −5 degrees. The possibility of magnitude equation and color error in the SRS catalog is discussed.

The reduction procedure used to transform the version of the SRS catalog based on the FK4/B1950.0 system to the version based on the FK5/J2000.0 system is given.

The new catalogue of Positions and Proper Motions (“PPM”) was compiled to replace the AGK3 and SAO catalogues as an astrometric reference on the northern celestial hemisphere. It provides more than 180 000 reference stars, is on the system of FK5 (J2000.0) and has a higher accuracy than the two older catalogues.

Some properties of PPM and its High-Precision Subset are presented.

The coordinate system of the Catalogue of Positions and Proper Motions (PPM) was constructed as an extension of the FK5 sytem to higher star densities and fainter magnitudes. Zonal deviations from the FK5 were minimised on scales of 7 × 7 square degrees via AGK3R and CMC catalogues. Compared with these catalogues, no magnitude equation was found for stars brighter than m(pg) = 10.5.

The entire series of Durchmusterung (DM) catalogs (Bonner, Southern, Córdoba, Cape Photographic) has been computerized through a collaborative effort among institutions and individuals in France and the United States of America. Complete verification of the data, both manually and by computer, the inclusion of all supplemental stars (represented by lower case letters), complete representation of all numerical data, and a consistent format for all catalogs, should make this collection of machine-readable data a valuable addition to digitized astronomical archives.

Between 1962 and 1972 the southern hemisphere has been covered by a 4-fold overlap pattern on 5820 plates, taken with a scale of 100 arcsec/mm and using a visual bandpass. More than 2 million pairs of x,y coordinates of the 2 exposures from 276259 stars have been measured at RGO with the GALAXY machine. The conventional plate adjustment, carried out at Hamburg Observatory, is now complete. About 150 000 primary stars will achieve a catalogue accuracy of about 0.06 arcsec at epoch of observation.

The rigorous FK4 and FK5 versions, using the SRS catalogue (C. Smith 1988), will be supplemented by preliminary new proper motions for those stars common with the SAOC. Finally, a rigorous block adjustment will be performed within the next year.

The goal of the present study is

1. (1) Completion of a general reduction of the Carte du Ciel astrophotographic catalogs, measurement of the positions in them on modern astronegatives, derivation of proper motions and compilation of a catalog of stars to magnitude 12.5.

2. (2) Copying the data of all 272 volumes of astrophotographic catalogues onto magnetic tapes (all together 8.5 million measurements).

3. (3) Measurement of modern astronegatives on FON-OBZOR-S and FOKAT-Y programs.

4. (4) Studies of the magnitude equation of the AGK3 catalog and other catalogs which will be presumably used as reference catalogs.

5. (5) Development and comparative studies of several methods of determination of equatorial coordinates on the basis of full information on astronegative measurements. Application of these methods to compilation of the catalog of 4 million stars.

The astrometric data bank ARIGFH will contain all relevant astrometric data on stellar positions and proper motions of stars from ground-based observations and space missions. For each star in the ARIGFH, the best available position and proper motion shall be derived. We rediscuss the accuracy of proper motions and positions of fundamental stars, resulting from a combination of data in the FK5 with the expected results from a revised HIPPARCOS mission. The FK5 data could be significantly improved even by rather degraded positions from a revised HIPPARCOS mission.

The work performed with the Belgrade Large Meridian Circle and the results obtained during the last twenty years are presented.

The U.S. Naval Observatory is building a measuring machine which will hold four PSS plates simultaneously and will automatically measure them, in a highly controlled environment, in about 12 hours. The images will be recorded by two camera subsystems with a scale of 0.9 arcseconds/pixel on a 0.33 × 0.26 degree CCD frame. The original glass plates of the PSS-II and glass copies of the PSS-I will be measured. This is a joint CALTECH-USNO program.

The project of fourfold coverage of the northern hemisphere of the sky by means of wide-angle astrographs was proposed in 1977 (Kolchinsky and Onegina, 1977). This project is known as FON (fotograficheskij obzor neba) — photographic survey of sky.

On the basis of the data observed with the Photoelectric Astrolabe Mark II of the Beijing Astronomical Observatory during the period from 1982 to 1988, residuals of 355 fundamental stars and 2921 catalogue stars are reduced and analyzed. The relations between the residuals V and the magnitude M, and the spectral type S of FK5 stars are discussed. The position corrections (PACP3—FK5), Δα′s and Δδ′s, from the double passage of the stars are determined. There are 910 in Δα and 696 in Δδ. The mean precisions of Δα′s and Δδ′s are ±3.9 ms and ±0″.067, respectively. Under certain conditions 328 Δα′s and 12 Δδ′s from the observed single passage of the stars are determined. Finally systematic corrections of PACP3—FK5 are given.

The positions and proper motions catalogue in the area near the NGP is presented.