Augmentation of VERITAS Telescopes for Stellar Intensity Interferometry

In 2018-2019 the VERITAS VHE gamma-ray observatory was augmented with high-speed optical instrumentation and continuous data recording electronics to create a sensitive Stellar Intensity Interferometry (SII) observatory, VERITAS-SII. The primary science goal of VERITAS-SII is to perform stellar diameter measurements and image analysis in the visible wavebands on a selection of bright (m< 6), hot (O/B/A) stars. The VERITAS Collaboration has agreed to the deployment and operation of VERITAS-SII during several days each month around the full moon period when VERITAS does not perform VHE gamma-ray observations. The VERITAS-SII augmentation employs custom high-speed/low-noise focal plane instrumentation using high quantum efficiency photomultiplier tubes, and a battery-powered, fiber- optic controlled High Voltage supply. To reduce engineering time, VERITAS-SII uses commercially available high-speed (250 MS/sec), continuously streaming electronics to record the time dependence of the intensity fluctuations at each VERITAS telescope. VERITAS-SII also uses fast (< 100 psec) data acquisition clock synchronization over inter-telescope distances (greater than 100 m) using a commercially available White Rabbit based timing solution. VERITAS-SII is now in full operation at the VERITAS observatory, F.L.Whipple Observatory, Amado, AZ USA. This paper describes the design of the instrumentation hardware used for VERITAS-SII augmentation of the VERITAS observatory, the status of initial VERITAS-SII observations, and plans for future improvements to VERITAS-SII.