The discovery of powerful X-ray sources associated with rich clusters of galaxies has led to the prediction that these clusters contain large amounts of hot gas. A necessary consequence of the existence of such gas is the Compton scattering of cosmic microwave background radiation by hot electrons (Sunyaev & Zeldovich 1972). As a result, some of the microwave photons are re-distributed with slightly higher energies and the effect may be observed as a small diminution in the background temperature. The effect is expected to be ≈0.5 mK for the gas clouds associated with cluster X-ray sources.

Previous attempts to detect this effect have not been successful. I present here the results of two further attempts; by Lake & Partridge using the 36’ telescope at Kitt Peak, and by Gull ’ Northover (1976) using the 25-m telescope at the SRC Chilbolton Observatory. Both groups used similar, twin-beam systems, comparing the temperature of the background radiation in the directions of various known X-ray clusters with that at points ≈15 arcmin distant. In order to minimize the effect of non-thermal background sources, it is desirable to observe at the highest frequency compatible with atmospheric stability. Gull & Northover, using a frequency of 10.6 GHz, accumulated 670 hours of observations, whilst Lake & Partridge, who did not have to contend with British weather, selected the more difficult frequency of 33 GHz, but could only manage 80 hours integration.

The preliminary designs for the LST and its instruments are now complete and present schedules call for the announcement of opportunity for the instruments to be issued during the summer of 1977 and launch of the telescope in late 1983. The astrometric instrument will be an integral part of the telescope’s fine guidance sensor system which is normally used to stabilize the telescope during exposures. Of the three fine guidance sensors, two are required to stabilize the telescope while the third is free to measure the relative positions of stars within a 40 square arc-min area. The instrument is designed to measure serially the relative positions of preselected stars with an accuracy of ±0″.002 (s.e.) to at least the 17th magnitude. The integration may be varied to achieve either higher accuracy or a fainter limiting magnitude.

In order to obtain very precise positions of the stars on a sphere, it seems absolutely necessary to go out the atmosphere.

1. 1 – We avoid atmosphere refraction with always uncertain corrections.

2. 2 – We avoid atmospheric turbulence. So the quality of the images is limited by diffraction and not by the seeing.

3. 3 – We avoid atmospheric absorption and diffusion limiting the possibilities on the faint stars. Moreover if we observe from an artificial satellite we have no bending of the instruments.

In view of very high precision (0″.001) on the directions from an artificial satellite, the difficulty is to obtain sufficient stabilisation in attitude. Also, it is necessary to measure the angles between the stars, instead of the directions relative to the satellite.

The mapping of a sphere by angular measurement requires precise measures of large angles; Indeed BACCHUS has shown how that is possible to obtain a precise sphere with relative positions and proper motions, but absolute parallaxes only by precise measures of angles near one value, 90°. Those measures are possible by superposition of two fields in one telescope. A special complex mirror (Fig. I) images the two fields at basic constant angle. We have only to make differential measures to add a small value to the constant.

The space astrometry projects being studied by the European Space Agency (ESA) promise an accuracy about ±0″.003 due to photon statistics for about 100 000 stars. Inherent limitations of the space astrometry projects and the future continued importance of ground based astrometry are outlined.