  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.
