Conclusions

Optical interferometry was suggested as an astronomical measurement technique over a century ago, but the technology required to exploit its full potential has only recently been developed. Present experimental arrays are already producing images with higher resolution than conventional terrestrial and space-based telescopes. Unfortunately atmospheric effects limit usable aperture sizes and integration times, making imaging impossible for objects fainter than about 10th magnitude²⁶ with present arrays. The greatest scientific potential for optical interferometry seems to be the study of local stars and star systems. Long baseline optical interferometry represents the only feasible technique for imaging stellar surfaces, and planned observations of gas giants and long period variable stars²⁶ are likely to improve our understanding of stellar astrophysics. The development of new arrays with higher sensitivity will open up new areas of research, such as the study of the environment around large black holes at the centre of galaxies.

The optical arrays currently capable of aperture synthesis imaging were designed as prototype instruments and are incapable of mapping complex sources. Future arrays must incorporate many more telescopes on a site with good atmospheric conditions if the full potential of astronomical optical interferometry is to be realised. Experience from present optical arrays suggests that in the near future interferometers sensitive to red or infrared wavelengths, operating with the complex visibility and closure phase measurement approach described in this report, are likely to have the greatest success. Maximum sensitivity is achieved with aperture diameters a few times greater than the atmospheric transverse coherence length²⁴, wide spectral channels and the minimum number of optical elements. Higher measurement precision can be obtained if slightly smaller apertures and narrow spectral channels are used. In order to measure the complex visibility on every baseline in an array of many telescopes the light beams from each telescope must be split many times, reducing the sensitivity of the instrument. For faint astronomical objects it may be necessary to treat an array as a set of smaller arrays with fewer beam-splitting operations on the light from each telescope. In future arrays, separate beam combining apparatus could allow simultaneous operation of several groups of three or four telescopes.

Optical interferometry and aperture synthesis have great potential as astronomical techniques. It is unlikely that terrestrial aperture synthesis arrays will ever have the sensitivity of large conventional telescopes, but long baseline interferometry represents a cost- effective technique for high resolution imaging of bright astronomical objects. The most ambitious proposals suggest arrays of terrestrial telescopes covering many kilometres, and arrays of orbital or lunar telescopes hundreds of kilometres in extent²⁷. These would permit a detailed search for planetary systems around other stars. These projects are still many decades in the future, but the experience gained with current arrays will be essential to their success.

Latest Long Baseline Interferometry News.

Previous Page / Abstract and Contents Page.

Return to Bob's Home Page.