In a first part, the present status of the HIPPARCOS mission is described. Despite the degradations and failures of gyroscopes, it is still hoped that a 4 1/2 mission duration will be reached. The first-year of data has been reduced by both FAST and NDAC consortia. For the best 46200 observed stars, the distribution of standard errors in positions has a maximum of 1.5 mas in latitude and 1.8 mas in longitude and the mean standard error for parallaxes is of the order of 3 mas. The comparison of results obtained by both consortia shows that the differences are small and quite consistent with the announced internal precisions. Magnitude measurements are precise to 0.02 magnitude for a 4 second observation. The precision to be expected for double star observations is also given. The main new result is that the magnitudes of the components are obtained with a few hundredths of a magnitude precision. This allows to devise a new method of mass determination based upon the parallax and a recalibrated mass-luminosity diagram. The parallax dependence of the results is much more favourable than in the case of the classical determination of masses using orbital motions.

Using the first 12 months of HIPPARCOS mission data already reduced, a synthesis solution was obtained by both CERGA and ARI teams. The reduction includes two steps. In the first one the origins of individual reference circles are determined, thus constituting the HIPPARCOS celestial reference frame. This solution uses a subset of some 30 000 bright and evidently single stars in order to minimize possible corruptive effects on the system. In the second step, the abscissa measurements of all observed stars are substituted in the system and then processed star by star in a least squares adjustment providing the astrometric parameters (positions and parallaxes).

The results of sphere solution and astrometric parameter determination are discussed. In this context error ellipses and internal errors are calculated. External errors are derived by comparison with suitable astrometric catalogues. Especially, for estimating the external errors of the HIPPARCOS measured FK5 stars the instrumental system was tied to the FK5 system by rigid rotations followed by comparisons of the positions with those of FK5. The mean internal errors of the HIPPARCOS positions of FK5 stars is 1.5 milliarcseconds (mas) while the standard deviation of the differences HIPPARCOS minus FK5 reaches 90 mas. For the 5022 measured parallaxes the mean internal and external errors are 3.8 mas and 16.7 mas, respectively. — The results of precision and comparisons with external data show that the accuracy predicted for the final catalogue can be reached.

Observations of 48 minor planets by Hipparcos should lead to a good determination of the inertial reference frame. Analysis of one year observations showed periodic effects in the o-c due to a rotation between the Hipparcos system and the inertial system. By extending this result to a 2.5 years simulation and by studying the impact of added ground-based observations, it appears that the orbital elements of the Earth and the parameters of a rotation of the system can be obtained with an accuracy better than 5 mas.

In addition to its astrometric capabilities the Hipparcos main detector proves to be a good phototometer. The main features of the photometric reduction applied to minor planets are outlined. The apparent magnitude measured by Hipparcos is transformed into absolute magnitude after corrections for the distance to the sun, the earth and for the phase effect, are applied. We show that the remaining signal contains information on the rotational properties of the planets.

A CCD modulation detector for a possible Hipparcos-2 mission is described. It is at least 1000 times more light efficient than the image dissector tube system used in the current Hipparcos mission. A beam combiner telescope with 29 cm aperture diameter could measure all 15 million stars brighter than 14th magnitude, providing accurate multi-colour photometry in addition to the astrometric parameters. Using two telescopes with different basic angles will further improve the rigidity of the resulting system of positions and proper motions. An accuracy of 0.5 milli-arcsec up to 10th magnitude is expected for a 2.5 year mission. The modulating grid is imaged on a CCD performing a ‘cophased integration’ of nine separate images of the whole field of view, each image representing a different part of the modulation period.

A satellite mission for accurate astrometry and multi-colour photometry is discussed, similar in principle to the ESA Hipparcos mission and here called the Roemer mission. The limiting magnitude will be about V = 18 mag while 13 mag is the limit of the present Hipparcos mission. Luminosities of stars up to 2 kpc away can be obtained, corresponding to a volume 10000 times larger than with Hipparcos. A mission of 5 years will provide an accuracy of 0.1 milli-arcsec at 12th magnitude for positions and parallaxes and 0.05 milli-arcsec for annual proper motions. This is achieved by a satellite using a mosaic of CCD detectors in the focal planes of two beam-combiner telescopes of 0.29 m aperture. The instrument is described and the expected performance with an input catalog of 400 million program stars is given.

This paper presents an overview of the current status of astrometry with the Fine Guidance Sensors (FGSs) on the Hubble Space Telescope. The FGSs have two astrometric modes of operation, one for positional (POS) and one for transfer function (TRANS) astrometry. The positional mode is intended for parallax and angular velocity work. Owing to optical aberrations in the Optical Telescope Assembly (OTA) (i.e., the main telescope optics) beyond the spherical aberration, and instabilities within the combined OTA/FGS optical system, it is undergoing additional feasibility review. Preliminary engineering tests showed changes in stellar pair positions of ∼ 100 milli-arc seconds which are far too large to allow for the expected ∼ 1 mas calibration. Alternate calibration procedures and data reduction algorithms are currently being developed to maximize the scientific potential of this mode which can be fulfilled at the ∼ 3 mas level.

The other operational mode-primarily intended for work on double stars but also usable for angular diameter determination-has proven more stable. Even the deleterious effects of the thermally induced solar array jitter have mostly been overcome. A large collection of reduction and analysis software is available to support this measurement technique (Lattanzi, Bucciarelli, Holfeltz, and Taff 1992) and its calibration is proceeding apace. Interesting scientific work from the FGSs on bright Hyades binaries and the highly eccentric double star ADS 11300 has already been published (Franz et al. 1991, 1992).

Finally, a new FGS Instrument Handbook (Taff 1992a) has been prepared which should minimize the complications of dealing with the General Observer Proposal Forms.

An origin of both accidental and systematic errors of Hipparcos star abscissae is studied in the frames of the great circle reduction algorithm, adopted in the Hipparcos mission. Systematic variations of the basic angle within one turn of the Hipparcos satellite may be caused by periodical irregularities of the thermal flow from the sun radiation inside the satellite optical unit, enclosed in a hexagonal envelope. This would lead to periodical errors in the star abscissae at certain periods. It is shown, that an oscillation of the basic angle with a period of 60 degrees is the most dangerous one, as it is amplifyed 4.5 times. Lindegren et al. (1992) found, that the sixth harmonic in the star abscissae residuals clearly dominated with an amplitude of 0.96 milli-arcsec. If this is caused by basic angle disturbances, it follows that the amplitude of such variation may be up to 0.21 milli-arcsec. It is pointed out, that the thermal basic angle disturbances are not the only reason to expect periodical errors in star abscissae. Random abscissa errors too are inevitably of strong periodical sort, the periods of the largest harmonics being 60, 30, 18.9 (2π/19), 14.4 (2π/25) and 11.6 (2π/31) degrees for a basic angle of 58 degrees. They arise from the spatial spectrum transformation of the white noise in the primary observational data due to so-called ‘non-rigidity’ of star abscissae. The non-rigidity term acts as a factor to the photon noise errors of observations for stars of certain magnitude. A radical way to diminish both accidental and systematic periodical errors in a future Hipparcos-like astrometric satellite would be to use two different basic angles (Makarov, 1992). The basic angles of 58 and 74 degrees were considered, as they were found to be an optimal combination. In that case an improvement of precision by a factor of 2.8 can be achieved with the same Hipparcos parameters, some part of which should be explained by doubling number of observations, that is 2. It may be concluded, that using two telescopes with different basic angles for space astrometry allows to increase significantly precision for a great deal of stars under the same conditions.

A principle for absolute determination of an object' position by means of LLMC with a CCD camera and the method for finding the error from the instrument and the CCD chip are described in this paper. The observational power and application of the LLMC with a CCD are also discussed.

The Flagstaff Astrometric Scanning Transit Telescope (FASTT) has been equipped with a new CCD detector, and further improvements in instrumentation have been made. An evaluation of its performance indicates that the telescope can determine accurate star positions, tied into the VLBI/VLA extragalactic reference frame, for stars as faint as V ∼ 17.5 mag. With further improvements, it is hoped this accuracy can be improved. A special scanning technique for stars at high declinations has been developed that allows scanning up to the pole. Positions tied into the extragalactic reference frame for FK5 and radio stars have been determined with the FASTT, and a comparison of these positions with their catalog values is given.

The experimental works for the development of the CCD micrometer to be used in global astrometry are coming to the final stage at the National Astronomical Observatory, Mitaka. Our goal is to observe and determine the precise optical positions of faint celestial objects deep to 16th mag with meridian circle. The so-called drift scanning method is adopted as the basic electrical architecture to detect and accumulate the photons from the target objects. Reported in this paper are the performances and some results of the test observations made by using the CCD micrometer equipped to Gautier meridian circle at Mitaka. The present CCD meridian circle was used in 1991 to determine directly the optical position of 3C273B in the FK5 system. Also reported are the test observations of the Seyfert galaxy NGC 1068.

Trigonometric parallaxes based on CCD observations are presented here for six southern very late-type M dwarf stars and three white dwarfs. The M dwarfs RG0050-2722, ESO207-61, MH2115-4518, MH2124-4228, and LHS3003 are among the very lowest luminosity stars known.

The DCMT (Danish-Chinese Meridian Telescope) is jointly developed by the Shaanxi Astronomical Observatory, Nanjing Astronomical Instrument Factory and Copenhagen University Observatory. The instrument was set up at the Shaanxi Astronomical Observatory in 1990 and put in testing operation in the same year. The design of this horizontal meridian circle is very compact and allows direct real-time monitoring of the instrument parameters. We have studied the stability of the instrument and internal refraction in the telescope tube.

Utilizing the photon counter to register the star through the 60° altitude and a microcomputer to control the telescope pointing, the automatization was attained in the observation of type II photoelectric astrolabe. The limiting magnitude of the improved astrolabe is up to 11.0 and the precision of single determination is ± 0.2 arc — sec. This automatic instrument is more powerful in improving the FK5 system and expanding the current optical celestial system into faint stars.