  We describe state-of-the-art MARCS-code model atmospheres generated for a
group of A dwarf, G dwarf, and late-G to mid-K giant standard stars, selected
to photometrically calibrate the Spitzer-IRS, and compare the synthetic spectra
to observations of HR 6688, HR 6705, and HR 7891. The general calibration
processes and uncertainties are briefly described, and the differences between
various templated composite spectra of the standards are addressed. In
particular, a contrast between up-to-date model atmospheres and previously
published composite and synthetic spectra is illustrated for wavelength ranges
around 8um (where the SiO Delta(v) = 1 band occurs for the cooler standards)
and lambda greater than 20um, where the use of the Engelke function will lead
to increasingly large discrepancies due to the neglect of gravity in cool
stars. At this point, radiometric requirements are being met, absolute flux
calibration uncertainties (1-sigma) are ~20% in the SH and LH, and ~15% in the
SL and LL data, and order-to-order flux uncertainties are ~10% or less.
Iteration between the MARCS model atmosphere inputs and the data processing
will improve the S/N ratios and calibration accuracies.
