Image reconstruction by means of speckle interferometry was successfully used to restore the intensity distribution of solar features and to investigate the morphology and dynamics of small-scale structures in active regions of the Sun. The observations were obtained with the Vacuum Tower Telescope (D = 70 cm, f = 46 m) at Observatorio del Teide, Tenerife, on May 17 and 20, 1991, from a plage region close to a sunspot near disc centre. Sequences of bursts consisting of 100 exposures were recorded with a broad-band filter centred at 550 nm (FWHM ≈ 10 nm, diffraction limit 0.2 arcsec). The pickup unit was a video CCD – system with an exposure time of 4 ms and a frame rate of three pictures per second. A description of the observing procedure and of the data handling can be found in de Boer et al. (1992). To obtain the complex Fourier phases speckle masking (Lohmann et al. 1983) was used. The speckle transfer function of the atmosphere was calculated indirectly using Korff's equation (1973). The Fried parameter r0 was estimated with the spectral ratio technique (von der Lühe 1984). This parameter was sometimes as large as 14 cm. With this the theoretical speckle transfer function could be determined for calculating the corrected Fourier amplitudes of the reconstruction. A new low pass filter, based on the reliability of each individual value in the Fourier plane, was applied to the amplitudes to suppress noise at high wavenumbers.

We have compared the images of two compact sources near the center of the Orion Nebula obtained with the VLA and HST. When the Hα image is scaled to the radio continuum we see similar objects. The VLA shows higher concentration of flux, reflecting the better resolution (FWHM=0.13″ versus 0.18″); but, does not show the extended cometary form of the sources due to the greater sensitivity of the HST.

We summarise the first year of operation of the Medium Deep Survey - a key project of the HST. Two fields in the LMC are discussed and some preliminary scientific results presented. We also comment on image deconvolution for the extragalactic fields observed as part of the Medium Deep Survey.

The resolution of close binaries and direct measurements of stellar angular diameters were the first achievements of astronomical interferometry (Michelson and Pease 1921). Fringe tracking interferometers are now capable of producing visibility measurements which are sufficiently sensitive and well-calibrated to make more sophisticated measurements possible. Results from current instruments include measurements of limb darkening, of the wavelength-dependence of stellar diameters, and of non-spherical stars, and observations with narrow spectral bands. This paper summarizes recent results from the MkIII interferometer, concentrating on single stars and their envelopes. More detailed descriptions of the instrument and the data reduction procedures are given by Mozurkewich et al. (1991).

The spectroscopic binary observing program of the Mark III Optical Interferometer has produced four published orbits and 22 preliminary orbits. The mean (formal) precision is 0.3% for measurements of sin i and 0.7% for measurements of the semimajor axis a. We list the stars for which we have obtained orbits or preliminary orbits and describe our data and uncertainty analysis techniques.

Eclipsing binary stars are of great importance in astronomy. Now, high angular resolution measurements of eclipsing stars with the Mark III Stellar Interferometer (Shao 1988) can determine not only all of the orbital parameters, but also the luminosities and color indices of the two components, and can yield precise information for the study of the stellar evolution models. Particularly, the combination of interferometric and photometric results provide a direct measurement of stellar effective temperature. In the case where the primary component can be resolved, the results from the optical interferometer then provide a reliable determination of the absolute system parameters. In addition, eclipsing binary stars are good candidates for comparison of different techniques, i.e., interferometric, photometric, astrometric, and spectroscopic, and for checking whether systematic uncertainties exist with certain techniques.

We have carried out lunar occultation observations at λ = 2.2μm at the WHT and Calar Alto telescopes of young stellar objects both in the Taurus and the Ophiuchus star forming regions. We report the discovery of 4 new binary systems: IW Tau and HK Tau/G2 in Taurus as well as GSS 35 and VSSG 14 in Ophiuchus; the projected separations are about 70 milliarcseconds (mas) for each of the Taurus objects, while for the Ophiuchus sources they are about 20 and 100 mas, respectively (20 mas correspond to 3 AU).

In this contribution we summarize some conclusions from the SCASIS project. The project, which concerns a multiple shearing pupil plane interferometer, was started in 1986 and was concluded in 1992 with the PhD thesis of de Vos.

The InfraRed Astronomical Satellite (IRAS) all-sky survey was designed and optimized for the detection of point sources. This allowed the survey to be conducted in the form of narrow strip scans with redundant coverage of the sky, but with non-uniform covering densities. The data shows in addition to point sources many sources of extended emission, which are best analyzed from images. However, the non-uniform coverage now forms a significant obstacle in the image (re-)construction. Low resolution images, such as in the Infrared Sky Survey Atlas, yield spatial resolutions of 5–10 times the IRAS telescope diffraction limit; HIRAS improves this to 1–2 times!

A Review is presented of the present status of astrometry towards defining a radio/optical reference frame.

The U.S. Naval Observatory Astrometric Interferometer (USNOAI) will be the dedicated astrometric sub-array of the new Navy Prototype Optical Interferometer (NPOI) at Lowell Observatory, which is being built in collaboration with the Naval Research Laboratory. The USNOAI will be in operation on Anderson Mesa, near Flagstaff, Arizona, in late 1993. The instrument will incorporate four siderostats which will be located in a Y-shaped configuration, and will feature state-of-the-art delay lines, a real-time zeroth-order fringe tracking algorithm, a full-array laser metrology system to monitor baseline motion, and a limiting magnitude of 10. The instrument will be capable of simultaneous observations on six baselines, and will produce star positions accurate to a few mas. With it, we will be able to maintain the HIPPARCOS reference frame with matching accuracy over the next few decades.

VLBI is at present the most accurate technique for measuring radiosource positions and the only method capable of high precision for a reasonable number of sources. The applications of VLBI astrometry in stellar, Galactic and extra-Galactic regimes are reviewed. In particular, substantial progress has been made in the last few years towards a global reference frame of extragalactic radiosource positions. The status of this frame, and of the link to the optical reference frames is also described.

We show preliminary results of three of the four radiosource pairs with angular separations ranging from 0.01° to 6° where we have determined such a separation with a typical fractional precision of 10−8 using phase delays corrected for structural and ionospheric contributions. In the radiosource 4C39.25 we measure a motion with respect to an external radiosource which is compatible with previously reported internal superluminal motion.

Using the north-south and east-south astrometric baselines of the MKIII optical interferometer on Mt. Wilson (each about 12 meters in length), we measured the positions of eleven EK5 stars with an accuracy of about 10 milliarcseconds (mas) in declination and 20 mas in right ascension.

The basic ideas of gravitational microlensing are discussed. Particular attention is given to the use of a gravitational lens as a (non-standard) telescope, to probe the very small scale structure of extragalactic sources. The recovery of information from a gravitational lens telescope is discussed, as well as the unique contribution which can be made by imaging with a gravitational telescope.

We have used the VLA to make images of the planets which are continuously illuminated by the high power planetary radar transmitter on the 70 meter antenna at Goldstone, CA. That instrument is capable of transmitting up to 460,000 watts of continuous power near 8.5 GHz. Radar imaging experiments became possible after the installation of the 8.5 GHz receivers on all VLA antennas by NASA for the encounter of the Voyager spacecraft with Uranus. A similar radar may be configured with the Australia Telescope (array) at Narrabri and the Goldstone 70 m at S-band which may be important for experiments on Venus whose atmosphere strongly absorbs at X-band. Highly successful experiments have been carried out at the VLA on Mercury, Venus, Mars, Saturn's rings and Titan, the giant satellite of Saturn.

The primary limitation to ground-based optical/IR interferometry is the turbulent atmosphere, which limits sensitivity by restricting the coherence volume, limits imaging accuracy by corrupting the fringe phase, and limits astrometric accuracy by corrupting the angle of arrival. Various advanced techniques can be used to circumvent these limits to some extent. Sensitivity can be increased with adaptive optics and laser guide stars, which should eventually be able to phase the individual apertures of an interferometer down to some cutoff wavelength, limited by tilt sensing. However, the sky coverage for cophasing the interferometer on an arbitrary object will remain limited at short wavelengths. For imaging, closure-phase techniques, well established in radio interferometry, will be used in next-generation instruments. However, for maximum sensitivity on extended objects, redundant arrays will be needed to cophase the interferometer. For astrometry, the limits to wide-field astrometry set by the atmosphere can be reduced somewhat with two-color techniques, but otherwise do not seem reducible by the techniques now being discussed. However, over narrow fields, the astrometric performance of an interferometer can be quite good. In space, without the corruptions of the atmosphere, the fundamental limitation is photon noise. However, technical issues such as metrology accuracy and practical issues such as maximum affordable baseline length will also limit performance.

Scientific objectives and some technical issues of Space VLBI (SVLBI) are considered. Two SVLBI missions under preparation – RadioAstron and VSOP – are briefly described.

The construction of a prototype telescope is now reaching the stage of mirror polishing and building the active cell. Three blanks have already been produced. Two polishing methods are tested, utilizing the stressing technics, or involving excentered polishing strokes. A working model of one leg of the telescope translator has been built.

An approach providing an estimate of the cosmological deceleration parameter qo from VLBI survey data is described.