Calibrating Snap

The SNAP (Supernova Acceleration Probe) mission's primary science goal is the determination of the properties of the dark energy. Specifically, observations of distant Type Ia supernovae will be used to measure the dark energy equation of state constant parameter, w(0), and time varying parameter, w(1), to a fractional uncertainty of 0.05 and 0.3 respectively. This places stringent requirements on the control of systematics and on the absolute color calibration of these supernovae. The overall calibration for the SNAP CCD and NIR imagers and spectrograph will be conducted through several routes. We envision employing a variety of well-studied stars, certainly including the HST spectrophotometric standard stars (and possibly the Sun) and performing indirect transfer calibrations that permit comparison with NIST irradiance standards to close the loop with fundamental MKS quantities. We discuss the basic issues and possible strategies in order to achieve similar to2-3% color errors over the wavelength range of from 350 to 1700 nm.