In-situ measurements of whole-dish reflectivity for VERITAS

The VERITAS array is a set of four imaging atmospheric Cherenkov telescopes (IACTs) sensitive to gamma rays with energies above 80 GeV. Each telescope is based on a tessellated mirror, 12 metres in diameter, which reflects light from a gamma-ray-induced air shower to form an image on a pixellated 'camera' comprising 499 photomultiplier tubes. The image brightness is the primary measure of the gamma ray's energy so a knowledge of the mirror reflectivity is important. We describe here a method, pioneered by members of the MAGIC collaboration, to measure the whole-dish reflectivity, quickly and regularly, so that effects of mirror aging can be monitored. A CCD camera attached near the centre of the dish simultaneously acquires an image of both a star and its reflection on a target made of Spectralon, a highly-reflective material, placed at the focus of the telescope. The ratio of their brightnesses, as recorded by the CCD, along with geometric factors, provides an estimate of the dish reflectivity with few systematic errors. A filter wheel is deployed with the CCD camera, allowing one to measure the reflectivity as a function of wavelength. We present results obtained with the VERITAS telescopes since 2014.