Sephira: stellar intensity interferometry with the MAST Array Telescopes

Abstract

Stellar intensity interferometry (SII) measures correlations in photon arrival times between independent apertures. Unlike amplitude interferometry, SII requires no optical beam combination: telescopes need only record photon timestamps locally, which are then correlated in post-processing. This eliminates the need for subwavelength optical path control and removes the central engineering barrier to kilometer-scale baselines, opening a path to sub-milliarcsecond angular resolution in the visible. By dispersing starlight onto single-photon avalanche diode (SPAD) arrays, photon correlations can be measured simultaneously across many independent spectral channels, yielding an order-of-magnitude sensitivity gain through spectral multiplexing. MAST, a spectroscopic telescope array conceived by the Weizmann Institute around the principles of simplicity, efficiency, and costeffectiveness, is a natural candidate for SII: its minimal optical design, high throughput, and independently mounted telescopes closely match the demands of a scalable SII array. The following paper presents Sephira, an SII experiment built around MAST, incorporating spectral multiplexing and sub-nanosecond timing synchronization via the White Rabbit protocol. We describe the design and initial laboratory results of the prototype, and outline our vision for a future dedicated facility: a modular array of portable, fiber-coupled units with flexible (u, v)-plane coverage over kilometer-scale baselines. Primary science goals include the study of rapidly rotating stars, Be stars, and interacting binaries, as well as precision visibility measurements at the sub-percent level to characterize stellar surface structure and limb darkening profiles, with broad relevance to exoplanet characterization and future missions such as the Habitable Worlds Observatory.