Program: GS-2015A-Q-40

Photometric calibration uncertainties are the dominant source of error in current type Ia supernova dark energy studies, as well as other forefront cosmology efforts, e.g., photo-redshift determinations for weak lensing mass tomography. Current and next-generation ground-based all-sky surveys require a network of calibration stars with 1) known SEDs (to properly and unambiguously take into account filter differences), and 2) that are on a common photometric zero-point scale. Our approved Cycle 20 and 22 HST programs afford us the ability to establish this essential network of faint (but within a magnitude range that will provide high signal-to-noise ratio observations during the course of current and future surveys), primary photometric standards, exploiting the well-understood energy distributions of DA white dwarfs, without the complications of observing through the time-variable Earth's atmosphere. A critical element is Gemini spectra, not for spectrophotometry, but to obtain the two parameters (temperature and log(g)) that determine the model SED. We can use the broadband HST photometry to set the overall flux scale for each source, and determine any applicable reddening. Thus calibrated, these standards can then be used as flux standards at wavelengths well beyond the range of HST and in any arbitrary, but defined passband. We propose to expand our network of DA white dwarfs with Gemini spectroscopy of new HST targets. This precision spectroscopic and photometric heritage from Gemini and HST will benefit essentially all existing and upcoming survey projects and directly address one of the current barriers to understanding the nature of dark energy.