The VLBI Space Observatory Programme (VSOP) is the first dedicated Space-VLBI mission. We report here on the planning for a second generation mission which builds on and extends the successful collaborations established for the VSOP mission, and which aims to improve both resolution and sensitivity by a factor of ∼10.

This presentation focuses on the problems associated with obtaining high quality images from a high orbiting space VLBI (SVLBI) mission. SVLBI intensifies and magnifies all imaging problems, making these problems clearer to understand, though much harder to solve.

Millimeter- and submillimeter wavelength interferometry is a powerful technique allowing imaging of dust and molecules in a multitude of astronomical environments. With the arrival of the Atacama Large Millimeter Array (ALMA), such studies will be possible with unprecedented sensitivity and a spatial resolution similar to the diffraction limit of large optical/infrared telescopes. In this paper, we mention a few aspects of (sub) millimeter astronomy and provide a short summary of ALMA's capabilities.

The FIR/SMM region is unique in the electromagnetic spectrum in its potential for vast increases in sensitivity and angular resolution, and, as a result, information vital to our understanding of the evolution of structure in the universe. About half of the luminosity in the universe is emitted in the far infrared. Evidence for this can be found both in the spectra of individual galaxies (Trentham et al. 1999) and in the cosmic FIR/SMM background found by COBE (Hauser et al. 1998; Fixsen et al. 1998; Dwek et al. 1998). JCMT/SCUBA observations suggest that “a large population of luminous, strongly obscured sources at redshifts ≲5 is missing from optical surveys” and could account for the background radiation (Blain et al. 1999). Future FIR/SMM measurements of these sources have the potential to reveal the luminosity history of the universe and will provide insight into the processes of galaxy and star formation and galaxy evolution.

First fringes with commissioning instruments and siderostats on both the Keck and VLTI facilities are expected for early 2001.

Interferometric techniques are at the forefront of modern astronomical instrumentation. A new generation of instruments are either operating or nearing completion, including arrays of small telescopes as well as the “big guns” (VLTI and Keck). A number of space interferometers for the detection of extra-solar planets are also being planned. I will review the current state of play and describe the latest developments in the field.

All 8-metre class telescopes will be equipped with adaptive optics systems in order to reach their highest angular resolutions. With the additional requirement for maximal sky coverage comes the mandatory use of a laser guide star. ALFA is an adaptive optics system using a sodium laser guide star, which is installed on the 3.5-m telescope at Calar Alto observatory. It has shown that it is possible to reach Strehl ratios better than 20% in the K-band. In this contribution we describe the design of the system, the observational trade-off necessary to use a laser guide star, and the performance that might be expected.

The major cornerstone of future ground-based astronomy is imaging and spectroscopy at the diffraction limit using adaptive optics. To exploit the potential of current AO systems, we have begun a survey around bright stars to study intermediate redshift galaxies at high resolution. Using ALFA to reach the diffraction limit of the 3.5-m telescope at Calar Alto allows us to study the structure of distant galaxies in the near-infrared at scales of 100-150 pc for z=0.05 and at scales 1.0-1.5 kpc at z=1. In this contribution we present the initial results of this project, which hint at the exciting prospects possible with the resolution and sensitivity available using an AO camera on the 8-m class VLT.

X-rays have tremendous potential for imaging at the highest angular resulution. The high surface brightness of many x-ray sources will reveal angular scales heretofore thought unreachable. The short wavelengths make instrumentation compact and baselines short. We discuss how practical x-ray interferometers can be built for astronomy using existing technology. We describe the Maxim Pathfinder and Maxim missions which will achieve 100 and 0.1 micro-arcsecond imaging respectively. The science to be tackled with resolution of up to one million times that of HST will be outlined, with emphasis on eventually imaging the event horizon of a black hole.

It is in the gamma-ray band, the shortest wavelength part of the electromagnetic spectrum, where diffraction limitations are least constraining, that one might hope for the highest angular resolution. The intrinsic limitations of the imaging techniques used in gamma-ray astronomy are reviewed and the feasibility of obtaining diffraction-limited angular resolution in this band discussed. It is argued that by accepting the necessity of extremely long focal lengths, sub micro arc second resolution along with enormously improved sensitivity should be achievable using diffractive lenses.

In the wake of the HIPPARCOS success four highly accurate space astrometry missions are now approved for launch within the next ten years. Stellar physics and galactic physics constitute the core science of these missions with deep impact on star formation and evolution, cosmic distance scales and age determination of the oldest stars. Summary data are provided in this paper to compare the goals and capabilities of these missions.

A personal view of the conference highlights.