Seeing Stars - Intensity Interferometry in the Laboratory and on the Ground
Abstract: Context Since the time of Tycho Brahe astronomy has been an observing science defined by the quality of its instrumentation. The introduction of primitive telescopes by Galileo in 1609 began a period of development of ever-larger telescopes with greater measuring precision. Refracting telescopes were soon overtaken technologically by reflecting telescopes. The epitome of this development is the building of the Extremely Large Telescope, ELT, in Chile today. However these single aperture telescopes are also reaching technological limits and about 100 years ago interferometric techniques began to be applied in order to access higher spatial resolutions. The state-of-the-art that such techniques have reached is demonstrated by the Very Large Telescope Interferometer, VLTI, again located in Chile. This technique is also rapidly approaching a technological limit as was recognised about 60 years ago by the development of a new technique known as intensity interferometry. Intensity interferometry is not in fact an interferometric technique but rests upon the correlation of photons from the same target star being observed in a number of different telescopes and the streams of photons being cross-correlated. The pioneering work of Robert Hanbury Brown resulted in the first intensity interferometer being built at Narrabri in Australia where the diameters of 32 stars were measured. No further development of this technique followed however and it remained in hibernation until just recently. Just over a decade ago the potential of intensity interferometry to go beyond the standards of so-called amplitude interferometry was recognised. A small number of people began to pursue the idea, notably at Lund Observatory, and in the past three years measurements have again started on the ground. Aim My project is a continuation of this initiative and has involved the building of a second generation laboratory Intensity Interferometer in Lund. The ultimate goal of such studies is to be able to image the surfaces of main sequence stars, currently beyond instrumental possibilities. Constructing the Mark-II Intensity Interferometer has comprised the use of ten small telescopes, or light collectors arranged in a two-dimensional array, observing an artificial star ~23m away in an isolated optics laboratory in the Lund Observatory building. This artificial star constitutes a small aperture (150μm and 200μm) illuminated by a green (532nm) laser, the light from which has been made non-coherent by scattering from a colloidal suspension. I have successfully investigated the use of other colloidal suspensions that give cleaner signals than the original milk solutions previously used. Ten telescopes give 45 baselines for a static target. Each baseline contributes one data point to an eventual image. In parallel to this development work I have pursued the acceptance of adding an Intensity Interferometry option to the ~100 telescope Cherenkov Telescope Array, the ground for which is currently being prepared in two sites in Chile and the Canary Islands. Such a facility gives access to ~5,000 baselines multiplied greatly by the transit of the star under observation. Images of such stars with 100,000 pixels and spatial resolutions ~25μas are quite feasible, given adequate signal to noise, S/N, levels. A number of other people have also pursued this development but my input has concentrated on the necessary political initiative to achieve this acceptance. My experience leading large international neutron sources has given me an insight into how to achieve such goals. Significant progress has been made with CTA itself but, pleasingly, smaller arrays of Cherenkov telescopes have also begun to make modifications that allow intensity interferometry measurements to be made and take the technique beyond what was achieved in Narrabri. As a further step I have proposed that outline design work should start now on a purpose-built Intensity Interferometer Array that goes beyond the secondary (or parasitic) use of CTA. I have sketched a strawman design that needs to be elaborated, and christened it SIITAR. It will not be completed for at least 50 years but one has to start somewhere with these cathedrals of science. Now is the time. Conclusions By extending the capabilities of the Mark-I Intensity Interferometer in Lund to a larger number of telescopes, a more intense artificial star, and a two-dimensional array, the Mark-II instrument now more closely matches the layout of the small intensity interferometer prototypes that are now operational in Arizona (VERITAS), on La Palma (MAGIC) and that is to be built on Tenerife (ASTRI). Accordingly we now have an instrument at Lund Observatory that can experimentally simulate layouts being proposed on other sites. Whilst the data quality is now significantly improved there are still problems that need to be addressed and further advances that can be made. At the same time the acceptance of an intensity interferometer science case as an option on the CTA large-scale array has made big advances. The perspectives, and the opportunities available show that the renaissance of this observational technique that is able to image the details of stars similar to our Sun, seen only as pinpricks today, is now within reach.
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