MRI Consortium: Development of Upgraded Stellar Intensity Interferometry Instrumentation for the VERITAS Observatory

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Very-high energy (VHE) gamma-rays are produced in the particle accelerations that occur in some of the most extreme environments in the Universe. As astrophysical messengers, their observation provides an important window to study the conditions and processes underway in these energetic events. This award supports the instrument development for upgrades to the mirror and camera systems of the VERITAS gamma-ray observatory located at Amado, AZ. In operation since 2007, the VERITAS imaging air Cherenkov telescopes are directly exposed to the Arizona elements, impacting the response of the 12 meter mirror systems over time. New developments in mirror resurfacing and advanced protective coatings make it possible to restore the reflectivity of the telescopes to nearly their original state, significantly enhancing the observatory’s sensitivity to VHE gamma rays across the full range of accessible energies. A complementary suite of focal plane instrumentation for enhanced collimation and bandpass filtering will also improve the effective mirror area observed by the telescopes by approximately a factor of two. Combined, the upgrades will significantly augment the on-going VERITAS observing missions of VHE gamma-rays from astrophysical optics, including real-time multi-messenger follow-ups, and the recently implemented novel stellar intensity interferometry (SII) program. The award provides unique opportunities for training of student and postdoctoral researchers in developing these new technologies that will enhance the existing VERITAS observatory and are critical to extending the techniques to the next generation Cherenkov Telescope Array (CTA). The mirror and focal plane upgrades will be installed on the existing four VERITAS telescopes in operation in Arizona. In cooperation with the original mirror manufacturer, the existing mirror coatings will be stripped and a new aluminum and hafnium coating will be applied to the mirror surface, with a quartz (SiO2) overcoat added for protection from the harsh Arizona elements. An accelerated program to rapidly recoat as many mirrors as possible is planned that will quickly bring the telescopes' reflectivity to 80-85%, providing a 30-35% average increase in light that will improve the observatory’s sensitivity to soft spectrum sources by approximately a factor of 2. It is anticipated that this overcoat technology will prove to be more durable and last longer (up to 5 years), reducing long-term maintenance for VERITAS and future instruments. The focal plane optics activities will allow for the development of the radial-gradient interference filter technology and a substantial reduction in focal plane noise. These developments may provide future transformational capacity for improving the sensitivity of large Imaging Atmospheric Cherenkov observatories, such as VERITAS and CTA, for future astrophysics research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.