Photon counting intensity interferometry in the blue at a 0.5 m telescope

Abstract

Abstract Intensity interferometry is a re-emerging interferometry tool that alleviates some of the challenges of amplitude interferometry at the cost of reduced sensitivity. We demonstrate the feasibility of intensity interferometry with fast single-photon counting detectors at small telescopes by utilizing a telescope of diameter of merely 0.5 m. The entire measurement set-up, including collimation, optical filtering, and two single-photon detectors, is attached directly to the telescope without the use of optical fibres, facilitated by the large area of our single-photon detectors. For digitization and timing, we utilize a time-to-amplitude-converter. Observing α Lyrae (Vega) for a total exposure time of 32.4 h over the course of six nights, an auto-correlation signal with a contrast of (9.5 ± 2.7) × 10−3 and a coherence time of (0.34 ± 0.12) ps at a signal-to-noise ratio of 2.8 is measured. The result fits well to preceding laboratory tests as well as expectations calculated from the optical and electronic characteristics of our measurement set-up. This measurement, to our knowledge, constitutes the first time that a bunching signal with starlight was measured in the B band with single-photon counting detectors. Simultaneously, this is to date the stellar intensity interferometry measurement utilizing the smallest telescope. Our successful measurement shows that intensity interferometry can be adopted not only at large-scale facilities, but also at readily available and inexpensive smaller telescopes.