Using Raster Scans of Bright Stars to Measure the Relative Total Throughputs of Cherenkov Telescopes

Gamma-ray astronomy at energies in excess of 100 GeV is carried out using arrays of imaging Cherenkov telescopes. Each telescope comprises a large reflector, of order 10 m diameter, made of many mirror facets, and a camera consisting of a matrix of photomultiplier pixels. Differences in the total throughput between nominally identical telescopes, due to aging of the mirrors and PMTs and other effects, should be monitored to reduce possible systematic errors. One way to directly measure the throughput of such telescopes is to track bright stars and measure the photocurrents produced by their light falling on camera pixels. We have developed such a procedure using the four telescopes in the VERITAS array. We note the technique is general, however, and could be applied to other imaging Cherenkov experiments. For this measurement, a raster scan is performed on a single star such that its image is swept across the central pixels in the camera, thus providing a statistically robust set of measurements in a short period of time to reduce time-dependent effects on the throughput. Photocurrents are measured using the starlight-induced baseline fluctuations of the pixel outputs, as recorded by the standard readout electronics. In this contribution we describe details of the procedure and report on feasibility studies carried out during the 2012-2013 observing season.