CCD transit telescopes are now determining star positions at faint magnitudes (i.e., down to V ∼ 17 mag) and with accuracies as good as ∼ ±150 mas. CCD parallaxes with errors ∼ ±1 mas are now being determined routinely in time periods less than 3 years.

A CCD/Transit Instrument (CTI) has produced relative astrometry with standard errors less than 2.6% of a 1.55 arcsecond pixel for stars with V ≤ 17. Additional astrometric studies with existing data are required to better understand the ultimate contribution these devices can make to our science.

The CTI is presently dismantled, awaiting a move to a new site. We briefly discuss the potential astrometric scientific returns from the exisiting data set, from a refurbished CTI, and from a similar device emplaced on the Moon.

Long baseline optical/infrared interferometers, such as the Mark III Stellar Interferometer1 on Mt. Wilson and the ASEPS-0 Testbed Interferometer2 on Palomar Mountain, California, have good capabilities for narrow-angle and wide-angle astrometry with very high precision. Using the Mark III Interferometer many spectroscopic binaries became “visual” for the first time. The measurement accuracy of angular separation is 0.2 mas, the smallest separation measured between two components is 2 mas, the maximum magnitude difference is 4 mag, and the smallest semimajor axis is 4 mas. Such high angular resolution and dynamic range have been used to determine stellar masses with precision of 2% and differential stellar luminosities to better than 0.05 mag for separations of less than 0.″2. For some binary stars, not only have the systems been resolved, but also the diameter of the primary component has been determined, yielding direct measurements of stellar effective temperature with high accuracy. For parallax determination, the precision is 1 mas or better and is unaffected by interstellar extinction. For wide-angle astrometry with the Mark III interferometer, the observation results yielded average formal 1σ errors for FK5 stars of about 10 mas. Presently a new infrared interferometer, the ASEPS-0 Testbed Interferometer on Palomar Mountain is under construction, and is being optimized to perform high accuracy narrow-angle astrometry using long baseline observations at 2.2 μm, with phase referencing for increased sensitivity. The goal is to demonstrate differential astrometric accuracies of 0.06–0.1 mas3 in order to allow for detection of extra-solar planets in the near future.

Present construction of large multi-element optical interferometers is based on experience with several pioneering instruments, a very successful one being the Mark III interferometer on Mt. Wilson, California. With it, over the last few years, some 26 spectroscopic binaries were resolved at separations as small as 3 milliarcseconds (mas) and orbital elements have been derived. In addition, the installation of vacuum delay-lines and multi-color fringe-detection allowed the derivation of dispersion-corrected geometrical delays of stars, which were used to determine relative stellar positions over wide angles with a precision of about 20 mas. We present and discuss recent results with respect to the determination of stellar positions, distances, masses, and luminosities.

The U.S. Naval Observatory Astrometric Optical Interferometer (AOI) began operation on Anderson Mesa, near Flagstaff, Arizona, in the autumn of 1994. The AOI incorporates four siderostats that are located in a Y-shaped configuration, and features a full-array laser metrology system to monitor baseline motion. The AOI incorporates state-of-the-art delay lines and a real-time fringe-tracking system. The AOI will have a limiting visual magnitude of 10, under typical observing conditions, and will produce star positions accurate to a few milliarcseconds (mas). With a planned operational lifetime of several decades, this instrument will be capable of maintaining the optical reference frame by improving the proper motions of thousands of the brighter HIPPARCOS stars through repeated observations.

The meridian circle is one of the most fundamental instrument in the field of astrometry where the astronomical objects are studied observationally for positions and their changes on the celestial sphere. At the Tokyo Astronomical Observatory (= National Astronomical Observatory since July, 1988) the Gautier Meridian Circle of 1903 was used until 1982 for various international meridian circle observations like SRS and NPZT programs, as well as for observations of OB stars, and the Moon and planets.

The fundamental system is currently based on the Dynamical Reference Frame as defined by the motions of the Earth and other Solar System objects. The link to this frame has traditionally been made by the observation of these objects and of stars in absolute transit circle programs. The zero points of the link are applied to quasi-absolute catalogs, and these are combined with the absolute catalogs to define the fundamental system. The individual positions and motions of the fundamental stars are then strengthened by incorporating differential catalogs reduced to the fundamental system. The system can be extended to higher densities and fainter magnitudes by further systematic reduction and combination of differential catalogs. The fundamental system itself, however, can only be extended through a planned series of observations resulting in absolute stellar positions over a range of epochs. A new fundamental catalog being compiled at the U.S. Naval Observatory is discussed and compared with the existing standard, the FK5.

One of the major uses of the Large Sky Surveys such as the Palomar and SERC Schmidt Surveys is the derivation of positions for faint objects discovered in the course of other surveys. The HST Guide Star Catalogue has proved to be of great help in deriving positions for such objects, but the well-known, and unavoidable, systematic errors in the GSC positions limit the accuracy of the final positions. In this Review, we will describe techniques that can be used to successfully transfer a reference frame from the bright FK5 stars to the small confines of a CCD chip containing only faint objects with the goal of maintaining an accuracy of approximately 0.1 arcsecond in the final positions.

The Lick Northern Proper Motion (NPM) Program will provide absolute proper motions (referred to faint galaxies), equatorial coordinates, and two-color photographic photometry for some 300,000 stars with 8 < B < 18 covering the 70% of the sky north of declination −23°. Part 1 of the NPM program (NPM1), recently completed, covers the 72% of the northern sky (899 of 1,246 fields) outside the Milky Way. Two catalogs result from NPM1: The NPM1 Catalog (Klemola et al. 1993a, Hanson 1993a) contains 149,000 stars. The NPM1 Reference Galaxy List (Klemola et al. 1993b, Hanson 1993b) contains 50,000 faint galaxies. Klemola et al. (1987, 1994, 1995) describe the NPM program. Hanson et al. (1994) describe the NPM1 Catalogs.

Large photographic catalogues still have their importance in modern astrometry. Old catalogues are used to derive good-quality proper motions. Present-day catalogues may be used to extend the Hipparcos and Tycho systems to fainter magnitudes.

The foreseen complexity of the the ESA Hipparcos mission led to the establishment of two independent scientific consortia (FAST and NDAC) for the parallel processing of all main mission data. The validation of Hipparcos data through internal and external checks and by interconsortia comparisons is outlined. Examples are given based on preliminary solutions. The various checks indicate that the overall accuracy and reliability of the Hipparcos Catalogue will be extremely high.

The Hipparcos satellite's star mapper gives photon counts in two spectral channels simultaneously, close to Johnson B and V. The transit times and the signal amplitudes for each star across two groups of four slits are derived and used for astrometry and photometry, respectively, and this constitutes the Tycho project. The present paper describes results of Tycho astrometric data processing, leading from the transit times to the astrometric parameters of the Tycho stars.

Some 30 months of Tycho observations, i.e. about 80 percent of the Hipparcos-Tycho mission, have been used to produce a working catalogue of Tycho positions, proper motions and parallaxes of a million stars. The external errors of this preliminary catalogue have been determined by comparison of 98 000 stars common with a preliminary, but much more accurate Hipparcos catalogue. External systematic errors of positions and annual proper motions are less than 0.5 milliarcsecond (mas) and the accidental errors per star are about 30 mas rms at V = 10.5 mag, the median magnitude of the catalogue. It is concluded that a satisfactory accuracy has been achieved.

As we approach the final processing of the observations carried out by HIPPARCOS, in particular for the double and multiple stars, it is possible to provide reliable statistics on the number of such objects detected and on the quality of the relative and absolute astrometry and photometry. About 24 000 stars have been recognized as non-single, including 11 000 already known as double and multiple before the mission and 13000 discovered by Hipparcos. Also, a subset of 16 000 stars among the 24 000 have been successfully solved for their relative coordinates (position angle and separation) with an accuracy in the range of 3 to 30 mas, including 7000 new double stars. I outline in this paper the principle of the internal recognition procedure and present some statistics on the solution.

The Hipparcos Input Catalogue -hereafter called HIC– (Turon et al. 1992a, Turon et al. 1994) contains the observing programme of the Hipparcos mission: 118 000 preselected stars, 48 minor planets and three satellites of major planets. These objects were selected on the grounds of more than 200 scientific programmes proposed by the world-wide astronomical community, and dealing with a large variety of astronomical and astrophysical topics. It contains the most up-to-date, comprehensive and homogeneous data and information related to these programme stars, collected during the years 1981-1990 by the INCA Consortium (Hipparcos INput CAtalogue consortium).

Internal error estimates and external verifications indicate that median errors for the 120000 stars in the Hipparcos Catalogue, for each of the five astrometric parameters, will be in the range 1–1.5 mas. This paper illustrates some of the statistical investigations that have been conducted so far, including comparisons with catalogues of ground-based positions and proper motions, the structure of the Hertzsprung-Russell diagram, results of the reanalysis of meridian circle and photographic plate data using the positions determined from the satellite, and expected results on double and multiple stars and photometric variability.

We briefly review the concept of double star measurement with HST Fine Guidance Sensors (FGS) in the Transfer Function (TF) Scan mode and give results for three calibration binaries observed with FGS3. Agreement among multiple observations indicates an astrometric precision of 1 millisecond of arc (mas) per observation. We compare measured angular separations with ephemeris values from orbits based entirely on speckle observations. This comparison shows that the accuracy of binary-star astrometry with FGS3 in the TF-Scan mode is 1 mas per observation. Multiple observations can be expected to produce relative positions of binary components at sub-millisecond of arc accuracy.

Hubble Space Telescope Fine Guidance Sensor 3 can generate sub-milliarcsecond precision parallaxes in eighteen months. We discuss the internal precision and external accuracy of our observations of Proxima Centauri and Barnard's Star. For some classes of targets Hubble Space Telescope will remain the parallax tool of choice for years to come. It can offer 0.5 mas precision. It will remain useful by satisfying urgent needs for quick results, by offering a 13 magnitude dynamic range, and by providing an unparalleled binary dissection capability.

We report on the highest angular resolution observation to date of the bright core, R136a, of the massive star cluster R136 within the 30 Doradus complex in the LMC. This visual observation was obtained with the interferometric fringe mode of operation of Fine Guidance Sensor No. 3 (FGS3) on board HST. Crowding and strong diffuse background from nebular emission make this a challenging observation.

The giant HII region 30 Dor has provided some of the best candidates for the most massive stars, like R136al. We provide evidence for a new component, R136a1B, within the previously known R136a1-a2 system with a separation of 80 mas (or ≈ 4000 au from a1 at the distance of the LMC), and δV = 1.1 mag fainter than the brightest component a1. Estimates from current evolutionary models of massive stars based on the new FGS photometry predict, after subtraction of a1B, that the present mass of R136a1 is 30 M⊙ with a main sequence progenitor of 60 M⊙. To date, this is the lowest direct estimate of the mass of R136a1.

The success of this difficult observation adds a new, unique feature to FGS3 and gives a much expanded, astrophysically very rewarding, role to the interferometer.

The astrometric capability of the Hubble Space Telescope Planetary Camera (WF/PC1) is investigated, motivated by a study of the internal velocity distribution of globular clusters. The astrometric accuracy of the HST PC will be determined ultimately by 1) the accuracy to which the aberrated images can be ‘centered’, and 2) the accuracy to which the distortions across the PC field can be modeled. A series of overlapping exposures of two clusters, NGC 6752 and M15, are utilized to examine these issues.

We have made use of maximum-likelihood image reconstruction to address the first issue, with good success. Reconstruction improves both the detectability and precision of the image centers. A preliminary exploration of the second issue, that of modeling the distortion across the PC field, is also presented, using positions derived from the multiple overlapping exposures.