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The angular size of a star is a critical factor in determining its basicproperties. Direct measurement of stellar angular diameters is difficult: atinterstellar distances stars are generally too small to resolve by anyindividual imaging telescope. This fundamental limitation can be overcome bystudying the diffraction pattern in the shadow cast when an asteroid occults astar, but only when the photometric uncertainty is smaller than the noise addedby atmospheric scintillation. Atmospheric Cherenkov telescopes used forparticle astrophysics observations have not generally been exploited foroptical astronomy due to the modest optical quality of the mirror surface.However, their large mirror area makes them well suited for suchhigh-time-resolution precision photometry measurements. Here we report twooccultations of stars observed by the VERITAS Cherenkov telescopes withmillisecond sampling, from which we are able to provide a direct measurement ofthe occulted stars' angular diameter at the $\leq0.1$ milliarcsecond scale.This is a resolution never achieved before with optical measurements andrepresents an order of magnitude improvement over the equivalent lunaroccultation method. We compare the resulting stellar radius with empiricallyderived estimates from temperature and brightness measurements, confirming thelatter can be biased for stars with ambiguous stellar classifications.

Direct measurement of stellar angular diameters by the VERITAS Cherenkov Telescopes