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R. C. Bohlin, S.S. Allam, S. E. Deustua, S. M. Kent, M. L. Lampton, N. Mostek, S. L. Mufson, M. Richmond, J. A. Smith, D. L. Tucker, B. E. Woodgate (STScI), SNAP Collaboration
The Hubble Space Telescope avoids all atmospheric complications and provides the best available spectrophotometry from the far-UV to the near-IR down to V~16. Any changes in instrumental sensitivities with time are small and well monitored. In particular, the now defunct Space Telescope Imaging Spectrograph (STIS) returned a wealth of calibration information via monitoring observations and observations of a variety of potential standards from Vega to stars as faint as V~16. Repeated observations of the same stars over the seven year STIS lifetime along with cross-checks of relative response compared to precision Landolt V magnitudes and compared to ACS photometry lend confidence that the relative spectrophotometry over the 0-16 magnitude range and over the 1150-10,000A wavelength range are correct to the ~1% accuracy required for the SNAP mission to achieve its goal of determining the dark energy equation of state. The absolute spectral energy distributions are determined as a function of wavelength by computed models of three pure hydrogen WD stars, while the absolute flux level is set by Landolt photometry relative to Vega in the V band. The WD models were originally for LTE, while the temperatures and gravities of the currently used preliminary NLTE models are under review.
Recent grism observations using the NICMOS instrument on HST have extended the wavelength coverage of a subset of 16 STIS standard stars to beyond two microns. However, a serious non-linearity of about 20% plagues the comparison of the STIS and NICMOS fluxes in their overlap region at 0.8-1.0 micron. ACS spectrophotometry for the same stars in the same wavelength band should help resolve this discrepancy.
Funding for this work is provided by NASA through the STScI, which is operated by AURA, under NASA contract NAS-5-26555 and by the Office of Science at the Department of Energy.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.