In this paper we develope an inertia! system of coordinates introducing a new observable: the arc between a pair of radio sources. We shall discuss how the conditional equation involving all the parameters is written in diferent systems when the observations are performed with a two and a three-element interferometers.

A system has been developed where data from a Mk3 VLBI processor can be read into the AlPS-package. Multiband and singleband delays can then be fitted globally by station. The technique has been used to phaserefence a background quasar to two radiogalaxies in the Abell cluster A2634.

Development of receivers and the new technique to perform global fringe fitting on Mk3 data have now made it possible to produce hybrid maps at 100 GHz.

Optical positions obtained with the Bordeaux automatic meridian circle (accuracy – 0.05”) are compared with VLA positions taken from the literature. Alignment of the FK5 and VLA frames are discussed, and other projects (HIPPARCOS, VLBI) are briefly presented.

We present results of a multi-frequency synthesis VLBI experiment which employed observations at 1622 and 1707 MHz. Combining data from two frequencies gave a factor ∼2 improvement in dynamic range over the single frequency images. We estimate map fidelity by comparing the images made separately from each frequency; while the dynamic range is ∼1800, the fidelity index is only ∼5.

The positions of circumstellar molecular masers relative to each other, to the central star, and to the compact extragalactic sources are of astrophysical and astrometrical interest. The SiO masers at λ = 7 mm are especially interesting as the maser spots are located very close to the photosphere of the star. For astrometry we would like to see a proper distribution of the maser spots and several epochs of observations in order to estimate the position of the central star and eventual motions of the spots.

The optical position of this star can be determined with ground-based astrometric facilities, and many of the stars showing SiO emission are in the Hipparcos catalogue. A link between the Optical and Radio Reference Frames is achieved in a second step relating the maser spot positions to the Extragalactic Reference Frame (EGRF) established with VLBI (Baudry et al., 1984).

In the course of implementing the use of nonrotating origin (Guinot 1979) in astrometric VLBI, for the transformation between celestial and terrestrial frames, we have compared the new approach with the classical one. We have shown that a difference exists between the two procedures at a few milliarcsecond level; this difference is due to the terms generally neglected when considering the equation of equinoxes in the classical representation of the Earth angle of rotation.

Differential 18-cm VLBI measurements of the position of PSR 1937+214 with respect to the quasar 1923+210 have been made to improve our knowledge of the offset between the planetary ephemeris reference frame and the extragalactic radio reference frame. Our preliminary frame-tie result (VLBI – Timing position) is +21 ± 2 milli-arcseconds in right ascension and +14 ± 9 milli-arcseconds in declination. We describe the data analysis methods used and compare our results with other radio interferometric position measurements of PSR 1937+214. Our results illustrate the value of combining the large collecting area of the Arecibo telescope with the Mk-III VLBI recording system.

The practical limits of phase referenced VLBI imaging are not well known. If it would be feasible to use target-reference separations of a few degrees, most of the sky would be accessible to the technique. Preliminary results at 6 cm using baselines of up to 1000 km are encouraging.

Most VLBI images have low dynamic range because they are limited by instrumental effects such as calibration errors and poor u, v-coverage. We outline the method used to make a new image of the bright quasar 3C345 which has very high dynamic range (peak-to-noise of 5000:1) and which is limited by the thermal noise, not instrumental errors. Both the Caltech VLBI package and the NRAO AIPS package were required to manipulate the data.

Interferometry at optical wavelengths is very similar to radio interferometry, once the fundamental differences in detectors are accounted for. The Mount Wilson Mark III optical interferometer has been used for optical interferometry of stars and stellar systems. Success with the Mark III has lead to the current program at the Naval Research Laboratory to build the Big Optical Array (BOA), which will be an imaging interferometer. Imaging simulations show that BOA will be able to produce images of complex stellar systems, with a resolution as fine as 0.2 milliarcseconds.

We describe an investigation of the science potential of the filled Mills cross orbiting, imaging, optical interferometer proposed by Synnott et al. and the constraints that science places on the design of the instrument. Imaging simulations were performed with this design in order to determine its capabilities in the area of spatial resolution, dynamic range, and sensitivity. In addition, specific astronomical objects of high interest were chosen as candidate program sources to determine what science was desired for these objects, what spatial resolution, etc. was needed to perform that science, and whether or not this interferometer design could do the science.

Lightning discharges radiate strongly at radio frequencies, and studies of the radiation provide valuable information on lightning breakdown processes. Interferometric techniques can be used to locate the numerous radiation events as a function of time during a discharge and to generate images of the developing lightning channels inside a storm, where they are obscured from view at optical wavelengths.

The Giant Metrewave Radio Telescope (GMRT) is being set up near Pune in India consisting of 30 numbers of 45-m diameter parabolic dishes. Twelve of these are being placed in a central array, about 1 × 1 km2 in size, and the remaining eighteen along three 14-km long arms forming a Y-shaped array. GMRT will operate at six frequency bands near 38, 153, 233, 327, 611 and 1420 MHz. It is expected to be completed progressively by 1993, and will be the world’s largest radio telescope operating at metre and decimetre wavelengths.

The Chinese VLBI Network (CVN) project is supported by the Astronomy Committee, Chinese Academy of Sciences. At present, the CVN project consists of three dedicated VLBI stations, two part-time VLBI stations, and a data analysis center. The sites and equipment of the CVN stations are listed in Tables I and II, respectively. The current status of the CVN data analysis center will be described in the last section of this paper.

The Very Long Baseline Array, currently under construction, is a dedicated VLBI array consisting of ten 25m diameter antennas and a 20 station correlator. The key design features that make it a flexible, general purpose instrument are reviewed and some details are provided of the array layout, frequency coverage and sensitivity.

Interferometric methods of studying the Cosmic Microwave Background (CMB) have some distinct advantages over the switched-beam techniques which have mostly been used. However, most existing interferometers are not well suited to CMB observations, for a variety of reasons. These include poor temperature sensitivity due to a low filling factor, and systematic effects which limit the maximum possible integration time. A new instrument, the Ryle Telescope, has been developed in Cambridge which has a high temperature sensitivity (120 μK in 12 h) and the ability to integrate for several hundred hours on the same field. It will be used to study the CMB on angular scales of a few arcminutes, with particular emphasis on the Sunyaev-Zel’dovich effect. A second instrument to study the CMB on angular scales of tens of arcminutes, the Cosmic Anisotropy Telescope (CAT), is also being developed.

In this paper the telescopes and signal processing systems of the Owens Valley Millimeter Array are briefly described.