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.

Popular Abstract

Public Outreach
Looking up into the sky at the stars shining so brightly in the darkness as generations of children have done, we began to wonder “What are those shining points of light that we call stars?” and “What in fact are we, as people on our planet Earth, compared to them?” The nursery rhyme “Twinkle, twinkle, little star, how I wonder what you are” has captivated the imagination of generations of children and adults alike. What we see as stars is in fact only starlight. The most elegant telescopes that mankind has built, whether on high mountain plateaus or in orbit around the Earth above the disturbances of our atmosphere, have only been able to glimpse rather blurred images of the almost unimaginably large giant stars; large enough to encompass the Earth’s orbit around our own Sun. The Sun is the only star that we can see up close and it has been studied with increasing precision for centuries. Its sunspot cycles and its coronal outbursts have fascinated mankind, and the rare eclipses have been both cultural events and opportunities for further scientific study. Mankind continues to find ways to study those stars that are similar to our own Sun; those stars that are sitting on the Main Sequence of stars that are middle-aged and mature, or younger and bright, or older and preparing to evolve into giant stars as they reach the end of their lives.
Here at Lund Observatory we have been working to develop a technique that would allow us to actually see stars rather than just the twinkling starlight that they emit. Others have gone before us of course and have built what are known as interferometers on the biggest telescopes, which have shown us the direction in order to achieve our goal of actually seeing stars. However even this technique does not get us far enough. Here in Lund we have been investigating how to couple many telescopes together to visualise stars, rather in the way that the images of the emissions around a black hole have been recently achieved. This spectacular image was produced by coupling together many radiotelescopes around the globe. What we are doing here in Lund is to extend this concept to the optical band - to the light that our eyes are sensitive to - using arrays of telescopes that are kilometers apart, not global in reach, but nevertheless which give a precision even greater than the images of that black hole. The work in the laboratory that we are doing – intensity interferometry - is under consideration to be installed as part of a scientific option for the more than 100 telescopes of the Cherenkov Telescope Array to be built on La Palma in the Canary Islands and in Chile. One day we will actually see stars, not quite in the way that we see our own star, but with much more detail then we could have imagined. This will allow us to place our own Sun in its right place within the Main Sequence of stars; to find the common elements between all stars and, perhaps more excitingly, to chart their differences. Like Tycho Brahe on the island of Ven more than 500 years ago we are - step-by-step - continuing our exploration of our stellar neighbours. “Twinkle, twinkle, little star!”

• author: Carlile, Colin
• supervisor: Dainis Dravins
• organization: Lund Observatory - Undergoing reorganization
• course: ASTM31 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Stellar Physics Instrumentation Interferometry Cherenkov Telescope Array Intensity Interferometry
• publication/series: Lund Observatory Examensarbeten
• report number: 2021-EXA173
• language: English
• id: 9054676
• date added to LUP: 2021-06-16 12:31:19
• date last changed: 2021-06-16 12:31:19