Almost 7 years ago after entering the University of Utah as a graduate student I read a paper by Stephan (now Tugdual) LeBohec (Univ. of Utah) and Jamie Holder (Univ. of Delaware) on the potential of Imaging Air Cherenkov Telescopes (IACTs) as optical intensity interferometers to study stellar systems with sub-milliarcsecond resolution. The technique seemed interesting, and the prospect of developing cutting edge astronomical instruments was more than enough motivation to pursue SII as my Ph.D. thesis topic.
IACTs are actually gamma-ray telescopes designed to image showers of faint Cherenkov ultra-violet light. Coincidentally, the requirements for observing these Cherenkov showers match those of an SII observatory, namely very large telescopes with nanosecond level time resolution. My advisor and PI of the project, David Kieda (Univ. of Utah), set me up in the laboratory developing an SII system using a high-speed digitizing system. Writing the data acquisition and analysis software was not so bad, the real challenge was dealing with correlated noise.
Since SII requires correlating intensities at approximately GHz frequencies over long timescales, it is extremely sensitive to radio-frequency (RF) interference, which could be seen by simply moving my phone closer to the digitizing system. Shielding the electronics helped, but spurious correlations persisted. To remove these effects, we measured correlations between orthogonal polarizations of light, which contained the spurious noise signal without coherence and subtracted this from correlations measured between parallel polarization filters. This scheme worked, and gave us the confidence to move the system onto IACTs.
A prototype instrument was built for the StarBase observatory (Grantsville, UT). StarBase was a unique experience as the observatory is on the site of a scuba diving location in the Utah West desert run by George and Linda Nelson who generously allowed the operation of a pair of 3-meter telescopes on their property! After sharing many grateful hours under the stars with T. LeBohec and several undergraduate students, we felt ready to scale the system onto VERITAS.
With assistance from the VERITAS observatory managers Michael Daniel (SAO) and Gareth Hughes (SAO) and on-site technicians, we installed the system onto two of the telescopes and performed engineering tests in Fall 2018. With some surprise, we quickly measured spatial coherence on two stars in January 2019. By May 2019, all four telescopes were equipped with SII capabilities. In December 2019, we were able to obtain three clear nights on the two stars, Beta Canis Majoris and Epsilon Orionis. Spatial coherence was seen for all six independent telescope pairs simultaneously, demonstrating the first intensity interferometer with more than two telescopes, and allowing us to determine the diameter of these stars with better than 5% precision.
The future of SII, in my opinion, is very bright. All major IACT arrays (as well as several groups using conventional optical telescopes) are now pursuing SII systems. Future observatories such as the Cherenkov Telescope Array could allow for SII systems on up to 99 telescopes over kilometric baselines enabling unprecedented angular resolution capabilities. The operation at blue wavelengths with large baselines opens a new phase-space of high angular resolution observations, thus complementing the exceptional observations of current generation amplitude interferometry observatories. Beyond angular diameters, SII has the potential to study more complex stellar features, such as limb-darkening and/or starspots, to resolve the elongation and constrain temperature gradients on rapidly rotating stars, characterize the orbital and component properties of close binary systems, and perhaps even generate images of stellar surfaces.
Header Image Credit: Tugdual LeBohec (University of Utah).