The goal of the new design and modernization of transit circles is complete automation (Pinigin and Shornikov, 1983; Hughes,1982; Requieme and Mazurier,1982). For example, all the essential steps of star observations and the determination of the instrument parameters have been automatized at the Tokyo and Brorfelde Observatories (Yoshizawa and Yasuda, 1982; Fogh Olsen and Helmer, 1978). Computer control was introduced for the automation of all the major operations of the Pulkovo horizontal meridian circle (HMC). This involves an automatic setting system (mirror setting), a circle reading system, two photoelectric eyepiece micrometers, meteorological data sensors and a rotating drive of the pendulum horizon (Figure 1). All these are controlled by a microcomputer of the “Electronica C5-12” type.

The Wanschaff vertical circle, set up at Goloseevo in 1949 was built in 1914. The instrument was originally equiped with four visual microscopes for reading the circle, with a visual eyepiece micrometer, and with two levels for the determination of the inclination of the vertical axis. After the instrument had been installed in a new pavilion in 1972, the visual microscopes were replaced by photographic devices, and a semi-automatic machine for measuring the circle readings with output on punched tape was developed. In 1973,the eyepiece micrometer was equiped with a precision potentiometer of the type which together with an automatic voltmeter of the type the digitizer micrometer readings punches them on tape. The other information is punched on cards by the operator at the data desk.

The azimuth of a meridian circle may be determined using stars within 1°–2° of the pole combining observations in upper and lower culmination of stars which have well known right ascensions. The right ascensions are determined photographically using only declinations in the reduction, thus making the azimuth determination independent of the right ascension system. The declinations of the pole near stars are determined with the meridian circle.

This paper discusses the effect of using a vacuum chamber with a horizontal entrance window to eliminate astronomical refraction, dispersion and irregular refraction in a telescope tube. Some engineering problems arising in connection with the entrance window are dealt with. The actual results obtained by vacuum photoelectric astrolabes and a vacuum photographic zenith tube are presented.

The modernization of the seven-inch transit circle is an important aspect of the envisioned Naval Observatory observing effort. This task has been carried out on the basis of the following desiderata:

1) The basic sensor should be photoelectric

2) Point sources observable at night down to at least m=10

3) Venus, Mercury and stars down to m=4 observable in the daytime

4) Mars, Jupiter, Saturn, Uranus, Neptune and the Moon observable

5) The Sun must be observable

6) The instrumental parameters: azimuth, level, collimation, nadir, and so on must be determined so as to allow absolute observations

7) The system should operate essentially autonomously, but an observer should be kept in the observing “loop”.

It appears that all of these objectives have been achieved by the system now to be briefly described.

The first two catalogues produced from test observations made with the Carlsberg Automatic Transit Circle (CATC) at the Copenhagen University Observatory in the period 1981/83 have internal mean errors of 0″.21 in right ascension and 0″.22 in declination. In 1983 the CATC was moved to the international Observatorio del Roque de los Muchachos where it will begin a programme of differential and fundamental astrometry in 1984.

The U.S. Naval Observatory (USNO) six-inch transit circle at Washington underwent extensive refurbishment during the interval between October 1982 and May 1983. Major items refurbished on the instrument included the micrometer, the clamp and slow motion arm, the screen system, and a circle. Support instrumentation refurbished included a new digital motor drive system, a new digital micrometer display system, and new environmental measuring systems. Finally, the telescope was completely repainted and the pavilion was extensively renovated.

On the basis of experience with RA determinations during the polar nights of 1974–1977 on West Spitzbergen and of RA determinations of the Sun, Mercury and Venus at high latitutes one can conclude as follows: At high geographical latitudes the polar night conditions are nearly optimal for absolute determinations of stellar coordinates; high altitude sites near the equator are best for day time observations. It is suggested that observatories be built for absolute position determinations on Spitzbergen, and on McMurdo on the Antarctic Continent. The link between observations in the northern and the southern hemispheres can be established through observations at a high-altitude equatorial observatory, where the Sun and the planets can also be observed.

More than a dozen years ago vacuum meridian marks were built for the Belgrade Large Transit Instrument with the purpose of completely keeping the air from interfering with the meridian mark readings. Information is presented on the favorable results achieved as regards both accidental and sytematic accuracy.

At Nikolaev, in 1978, a system for the improvement in the stability of the light path between the transit instrument and the meridian mires was designed. Its purpose is to improve the accuracy of the azimuth determination and that of the collimation. This system consists of twelve sections of 8 mm thick steel tube with 160 mm diameter, each nine meters long. These tubes are lying in special steel cradles which rest on brick support columns spaced 8 meters apart, and these can be moved vertically and horizontally by 100 mm and 50 mm, respectively. Thus it is possible to align the axis of the tube exactly.

Tube refraction of the order 0″.3 inside a meridian circle is a source of systematic errors that has not been noticed before. Natural convection due to cooling at night or heating during day will create primarily vertical temperature gradients in the air. This is shown by experiments at a collimator where typically 1 K/m was found inside the tube even when the gradient outside the tube was much smaller. A method of tangential ventilation of the tube was used to eliminate the gradients. It is shown that the resulting effect on seeing or image motion will be to the better when the ventilation system is properly designed, and the design for a conventional meridian circle and for a Glass Meridian Circle is discussed.

The automatic declination reading system implemented on the Bordeaux meridian circle with a new divided circle is shortly presented. The determination of the division errors by the Benevides-Boczko method was carried out in December 1982 and in March 1983: the standard deviation between the two sets of corrections is about 0.015″.

This article describes a new method by which azimuth and the instantaneous latitude of a meridian-prime vertical-transit instrument can be determined absolutely in mid-and low- latitude areas, and discusses some experimental observations obtained on a remodeled ZEISS transit instrument. Requirements for the development of a new type of transit circle along the lines of this new method are also presented.

In meridian observations, the main error sources of the declination observation reside in determination of the atmospheric refraction, the flexure of the telescope and the division errors of the graduated circle. In order to obtain higher accuracy in the declination observation, besides a full automation device, the photoelectric meridian circle at Tokyo Astronomical Observatory (Tokyo PMC) is equipped with photoelectric devices for circle reading and a zenith mirror in addition to the conventional mercury horizon. We report here on a preliminary determination of the horizontal and vertical flexures, and of the division errors of the glass circle.

Mao et al. (1983) have originally stated that in order to record the transit times of a star at meridian transit as well as at prime verticale transit the chassis of the instrument would have to have two pairs of supports. In my opinion, it may be difficult to develop such an instrument with the required aperture of 20 cm and a focal length of over two meters, especially when the declination system must be very stable.

In the course of the cooperation between the Nikolaev and the Pulkovo Observatories, a photoelectric micrometer, similar to that proposed by E. Høg, was designed and installed on the Repsold meridian circle.

Results are presented of the comparison of five Tokyo PZT cataloques with the AGK3 and three other independet catalogues obtained with meridian circles.

The HIPPARCOS satellite is an approved project of the European Space Agency since 1980. A sketch of the past and future development of the project is given. The resulting astrometric parameters and magnitudes of 100 000 stars should be available circa 1993. Many astronomical groups are involved in the preparation of the mission together with ESA bodies and industry. Their functions and mutual interfaces are described.

The main features of the satellite are presented. While observing out of the atmosphere removes most of the effects that limit the accuracy of ground based astrometry, new causes of systematic or random errors appear in an instrument designed for a millisecond of arc level of precision. The main limitations that have been identified are discussed: basic angle unstability, mechanical jitter, grid irregularity, diffraction chromatism, shape of the image dissector sensitivity profile. The consequence is that the presently expected basic measurement precision is limited not only by the photon count errors. However, all these effects are taken into account when assessing a final accuracy of 0″.002 for the astrometric parameters of a 9-th magnitude star.

The raw data produced by the European Space Agency's astrometry satellite HIPPARCOS – mainly photon counts from the primary detector (IDT) and star mappers – will be analysed in full by two independent reduction consortia, FAST and NDAC. The aim is however to generate, by 1993, a single catalogue of positions, proper motions, and parallaxes for the 105 programme stars, with additional information on photometry, multiple stars, and minor planets. This review outlines some practical and theoretical features of the data reductions.

The various aspects of the preparation of the Hipparcos Input Catalogue are presented here. The astrometric parts of the work are pointed out in more details, especially the new observations and measurements already undertaken. The astrometric proposals submitted for observation with Hipparcos are also presented, as they will allow the link between Hipparcos, Space Telescope and ground-based astrometric programs.