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As the precision frontier of large-area survey astrophysics advances towards the one millimagnitude level, flux calibration of astronomical instrumentation remains an ongoing challenge. We describe initial testing of silicon solar cells as large-aperture precise calibration photo- diodes. We present measurements of dark current, linearity, frequency response, spatial response uniformity, and noise characteristics of the Sunpower C60 solar cells, an interdigitated back-contact 125mm x 125mm monocrystalline solar cell. We find that these devices may hold promise as large-area flux calibration sensors, and that further analyses over a broader range of operating conditions are necessary. Flux calibration remains a primary source of systematic uncertainty in the use of type Ia supernovae (SNe Ia) as probes of the history of cosmic expansion [1, 2, 3, 4]. The wavelength-dependent throughput of the observing instrument is the most immediately accessible and separable contribution to this systematic error. One approach for flux calibration is to invoke models of photon emission spectra vs. wavelength for a simple stellar atmosphere, white dwarf stars being the most popular [5]. This calibration method includes contributions from the instrument throughput, but is also affected by uncertainties in Galactic and atmospheric extinction, and other systematic effects. A supplemental approach that isolates the instrument throughput is to use well-characterized sensors as the metrology standard for relative flux determination [6, 7, 8, 9, 10]. In this approach a well-calibrated photodetector, known to better than a part per thousand, [11] is used to map out the instrument’s relative sensitivity vs. wavelength. Conventional photon- detectors (photodiodes, CCDs, etc) have collection areas no larger than a few square centimeters. Such small collection areas are inadequate for some mod- ern imaging applications that depend on the calibration of a large-diameter optical beam. The Large Synoptic Survey Telescope [12] (LSST) project intends to use a collimated, monochromatic beam (a Collimated Beam Projector, or CBP), to sequentially illuminate portions of the optics, and a calibrated silicon photodiode to monitor the flux [13, 14]. The LSST team plans to use the CBP to measure instrument transmission as a function of photon wavelength and source position. One metrology challenge of this approach is to measure the flux emanating from the CBP, which has an exit pupil diameter of 240 mm, about twenty times larger than the diameter of typical silicon photodiodes. The LSST team is considering several solutions to this unsolved problem, including changing the projector beam focus to reduce the spot size, collecting the light with a focusing concentrator, using an integrating sphere with a 250mm port, or scanning the exit beam across one or more standard calibrated photodiodes. Each of these approaches, all suboptimal, are born of a perceived need to measure a large light source with a small detector. In this paper we consider the possibility of using an array of high-efficiency solar cells as a full-aperture sensor for calibrating the LSST CBP and other large-diameter internal calibration light sources. In Section 1, we describe the general architecture of an interdigitated back-contact monocrystalline silicon solar cell and introduce the C60 solar cells that we study. In Section 2, we describe the methods and results of our measurements. We conclude in Section 3, where we summarize the results of our experiments and discuss what additional steps should be taken to fully assess the prospect of creating a precision photometric calibrator composed of solar cells.
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