Cherenkov Telescopes as Optical Telescopes for Bright Sources: Today's Specialised Thirty Metre Telescopes?

Imaging Atmospheric Cherenkov Telescopes (IACTs) use large-aperture (~ 10 - 30 m) optical telescopes with arcminute angular resolution to detect TeV gamma-rays in the atmosphere. I show that IACTs are well-suited for optical observations of bright sources (V <= 8 - 10), because these sources are brighter than the sky background. Their advantages are especially great on rapid time-scales. Thus, IACTs are ideal for studying many phenomena optically, including transiting exoplanets and the brightest gamma-ray bursts. In principle, an IACT could achieve millimagnitude photometry of these objects with second-long exposures. I also consider the potential for optical spectroscopy with IACTs, finding that their poor angular resolution limits their usefulness for high spectral resolutions, unless complex instruments are developed. The high photon collection rate of IACTs is potentially useful for precise polarimetry. Finally, I briefly discuss the broader possibilities of extremely large, low resolution telescopes, including a 10" resolution telescope and spaceborne telescopes.