Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.9818s&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #98.18; Bulletin of the American Astronomical Society, V
Mathematics
Logic
Scientific paper
The SNAP mission will study Type Ia supernovae (SNe) to probe the dark energy content of the Universe. Photometric calibration is critical to the success of the mission. Kim & Miquel (2006) computed the impact of various levels of band-to-band photometric zero-point offsets on the derived cosmological constraints and determine the Type Ia method constraints reach full effect when the relative band photometry is accurate to 1% or better. Poorer photometry leads to larger scatter in the derived redshift-magnitude diagram for SNe and weakens the constraints on dark energy models. We outline the photometric calibration program designed to provide sensitivity calibrations with systematic errors no greater than 1-2% for both the SNAP spectrograph and imager.
Currently, the best absolute flux standards covering the 0.4-1.7µm range are the HST hot (T$_{eff}$=30,000-60,000K) pure hydrogen WDs. NLTE models of these WDs determine the shape of their SEDs, while precise Landolt V band photometry sets the overall absolute flux level. Models of A stars and K giants along with solar analog stars are also commonly used for flux standards; but these alternative approaches are not as precise as the models for pure hydrogen hot WDs. SNAP must observe the WDs in both the spectrograph and the imaging modes with an exposure time uncertainty of <1%.
The achievement of <1% errors in the transfer of the flux calibrations from the fundamental standard stars to the SNAP instrumentation will be demonstrated by the repeatability level of multiple observations of the SNAP fundamental standards. We discuss the uncertainties in the fundamental standards, in transferring the calibration to the SNAP instrumentation, variations over the FOV, scaling (linearity) factors, and instrument/spacecraft factors (shutter timing, filter bandpasses, etc.). We include a discussion of the impacts to the calibration program if the systematic error requirements are increased from 1% to 0.5%.
Allam Sahar S.
Bohlin Ralph Charles
Deustua Susana E.
Kent Stephan M.
Lampton Michael L.
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