  We have used a precision calibrated photodiode as the fundamental metrology
reference in order to determine the relative throughput of the PanSTARRS
telescope and the Gigapixel imager, from 400 nm to 1050 nm. Our technique uses
a tunable laser as a source of illumination on a transmissive flat-field
screen. We determine the full-aperture system throughput as a function of
wavelength, including (in a single integral measurement) the mirror
reflectivity, the transmission functions of the filters and the corrector
optics, and the detector quantum efficiency, by comparing the light seen by
each pixel in the CCD array to that measured by a precision-calibrated silicon
photodiode. This method allows us to determine the relative throughput of the
entire system as a function of wavelength, for each pixel in the instrument,
without observations of celestial standards. We present promising initial
results from this characterization of the PanSTARRS system, and we use
synthetic photometry to assess the photometric perturbations due to throughput
variation across the field of view.
