SNAP Calibration Program Steps to Spectrophotometric Calibration 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‘ or w a, 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 performing indirect transfer calibrations that permit comparison with NIST irradiance standards to close the loop with fundamental MKS quantities. We present the basic issues and possible strategies in order to achieve 2% uncertainties in color calibration over the wavelength range from 350 to 1700 nm. The science requires observations of nearby and distant SNe Ia. At redshift z=0, SNe Ia restframe B-band flux is measured in a B filter (Figure 1a) and SNe Ia are bright at peak, m~19 mag At high redshifts, to z=1.7 SNe Ia restframe B-band flux shifted to longer wavelengths (Figure 1b) SNe Ia are faint at peak, m~25 mag To standardize SNe Ia, we only need to know the flux RATIO in any two filters: e.g. f B /f J equivalent to the COLOR: e.g. B-J = -2.5 log f B /f J Just need to know the SLOPE of the SED. The problem to solve: how to accurately measure this ratio for faint objects, and in the near infrared (NIR). What effect does calibration error have on the uncertainty in w 0 and w  ? Assuming uncertainty solely in the temperature of the calibrator: A) A single calibrator gives rise to a negligible increase in systematics: the extinction correction effectively corrects for a one- parameter miscalibration (Figure 3a). B) Two calibrators: at T=35000 and at T=6000, calibrating different wavelength regimes. The uncertainty in w 0 and w  increases by approximately 20% for uncertainties in the temperature of ~ 3%. If there is miscalibration of the spectra (i.e. mismatch in the overlap region) this is inferior to a single star spanning the entire spectral range (Figure 3b). 1.Wide range of target and standard star brightness (Figure 2a) filter + detector combinations (Figure 2b) –4 3x3 HgCdTe arrays –NIR filters: 12 each of J-, H-, K-like –4 3x3 CCD arrays each with 4 filters –Optical filters: 24 each U, B, V, R, I, Z 3.2-channel integral field spectrograph –2 optical and 2 NIR detectors 1.Identify good calibration standards 2.Establish standard star network 3.Construct calibration error budget –(iterative process with simulation and instrumentation teams) 4.Investigate optimal telescope platforms for fundamental calibration, tying standard star network to NIST standards: e.g. balloons, space & ground 5.Develop pre-launch and in-orbit calibration strategies 6.Develop on-board calibration observing program –Study impact of on-board calibration on observatory management observing strategy, hence SN program observatory design 7.Design spectrophotometric calibration experiment 8.Develop realistic time, cost and manpower estimates for –pre-launch calibration program –in-orbit when calibration included in observatory management How well are fundamental flux standards calibrated? Uncertainties in calibration transfer (“error transfer”) –from fundamental flux standards to spectrophotometric primaries –from primaries to secondary standards –from secondary standards to science targets Requirements on flat fields Uncertainties introduced by other effects, for example –filter properties –position on focal plane –numbers/types of standards Color calibration Zero point (sets scale) Secondary star field calibration Flatfields Primary standards calibration Aperture corrections Scattered light Detector Telescope linearity intra- pixel OpticsFilter Instrument: Imager & Spectrograph band pass shifts uniformity Error budget accounts for e.g. : How do calibration transfer errors propagate down the network? How well do filters+detectors, integral field spectrograph need to be calibrated? Flat field requirements? Figure 2b System throughput & bandpass of each of the SNAP filters shall be known to sufficient precision to predict the relative count rate in each of the bands to 1.5% rms and therefore the ratio of any two bands (i.e. the color) to 2% given a flux spectrum of a SN Ia with redshift 0.3 < z < 1.7. Systematic uncertainties are the limiting factors. Implies 2% maximum in systematic uncertainties in the calibration error budget. HgCdTe Spectrograph Spectrograph port CCDs Guider ~50 mags Figure 2a WD flux standard KIII flux standard NIST Lamp SNe Ia Z=1.7 Z=0 Figure 1a Figure 1b SN Ia Restframe B-band Peak Spectrum SN Ia at z=1.4 redshifted B-band Peak Spectrum Figure 3a. Figure 3b Calibration Requirements Abstract The Challenge Need to Check Science Driver: w 0 andw a Error Budget Framework