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The calibration hardware system of the Large Synoptic Survey Telescope (LSST)is designed to measure two quantities: a telescope's instrumental response andatmospheric transmission, both as a function of wavelength. First of all, a"collimated beam projector" is designed to measure the instrumental responsefunction by projecting monochromatic light through a mask and a collimatingoptic onto the telescope. During the measurement, the light level is monitoredwith a NIST-traceable photodiode. This method does not suffer from stray lighteffects or the reflections (known as ghosting) present when using a flat-fieldscreen illumination, which has a systematic source of uncertainty fromuncontrolled reflections. It allows for an independent measurement of thethroughput of the telescope's optical train as well as each filter'stransmission as a function of position on the primary mirror. Second, CALSPECstars can be used as calibrated light sources to illuminate the atmosphere andmeasure its transmission. To measure the atmosphere's transfer function, we usethe telescope's imager with a Ronchi grating in place of a filter to configureit as a low resolution slitless spectrograph. In this paper, we describe thiscalibration strategy, focusing on results from a prototype system at the CerroTololo Inter-American Observatory (CTIO) 0.9 meter telescope. We compare theinstrumental throughput measurements to nominal values measured using alaboratory spectrophotometer, and we describe measurements of the atmospheremade via CALSPEC standard stars during the same run.

Testing of the LSST's photometric calibration strategy at the CTIO 0.9 meter telescope