Reference Calibration for Precision Dark Energy Measurement

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Uncertainties on photometric standards now constitute the dominant uncertainty on our knowledge of dark energy. Additionally, they constitute a significant systematic for measurements of stellar population in galaxy cluster counts, and upcoming photometric redshift surveys measuring growth of structure. There are prospects for improvement in uncertainties from standard star flux, but the traditional techniques of measurement of stellar flux from above the atmosphere suffer from basic and inherent problems: the variability of all stellar sources, and the difficulty of creating a precisely calibrated, cross-checked, and stable platform for observation above Earth's atmosphere. We are undertaking a program of research to understand and profoundly reduce the many astrophysical and cosmological uncertainties due to photometry. Presently, together with colleagues at Toronto, McGill, NRC-INMS, and U.S. institutions, we have designed and constructed payloads for high-altitude balloons containing both monochromatic light sources and radiometric calibration. Our balloon flights have begun to overfly observatories, starting with our initial test flight in New Hampshire in August 2011, and continuing with a campaign of flights over the Univ. of New Mexico campus observatory and Mt. Hopkins in Arizona in 2012, and the following year with the CTIO and Las Campanas observatories in Chile. We have constructed and are the responsible party for the payload optics, including the light source, integrating sphere, and regulated power supply. As well as our initial free-flying balloon payload, both a teststand, and a calibrated payload on a tethered weather balloon, are taking data on an optical table and with the new 32" UVic telescope observatory, respectively. Together with an RTI grant with Canadian colleagues submitted separately, we presently request funding for an MSc student to analyze the data from the overflights, as well as from our calibrated lightsource observations at UVic. The longer-term goals of the program are calibrated sources on a nanosatellite, as well as on a controlled high-altitude miniature blimp, to fly directly "in front of" SNIa, and allow observatories across the world to reach the then-technology-limited precision of 0.01% photometry.