ACCESS: Enabling an Improved Flux Scale for Astrophysics

Improvements in the precision of the astrophysical flux scale are needed to answer fundamental scientific questions ranging from cosmology to stellar physics. The unexpected discovery that the expansion of the universe is accelerating was based upon the measurement of astrophysical standard candles that appeared fainter than expected. To characterize the underlying physical mechanism of the "Dark Energy" responsible for this phenomenon requires an improvement in the visible-NIR flux calibration of astrophysical sources to 1% precision. These improvements will also enable large surveys of white dwarf stars, e.g. GAIA, to advance stellar astrophysics by testing and providing constraints for the mass-radius relationship of these stars. ACCESS (Absolute Color Calibration Experiment for Standard Stars) is a rocket-borne payload that will enable the transfer of absolute laboratory detector standards from NIST to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of R = 500 across the 0.35-1.7 micron bandpass. Among the strategies being employed to minimize calibration uncertainties are: (1) judicious selection of standard stars (previous calibration heritage, minimal spectral features, robust stellar atmosphere models), (2) execution of observations above the Earth's atmosphere (eliminates atmospheric contamination of the stellar spectrum), (3) a single optical path and detector (to minimize visible to NIR cross-calibration uncertainties), (4) establishment of an a priori error budget, (5) on-board monitoring of instrument performance, and (6) fitting stellar atmosphere models to the data to search for discrepancies and confirm performance.