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We present the stellar calibrator sample and the conversion from instrumentalto physical units for the 24 micron channel of the Multiband Imaging Photometerfor Spitzer (MIPS). The primary calibrators are A stars, and the calibrationfactor based on those stars is 4.54*10^{-2} MJy sr^{-1} (DN/s)^{-1}, with anominal uncertainty of 2%. We discuss the data-reduction procedures required toattain this accuracy; without these procdures, the calibration factor obtainedusing the automated pipeline at the Spitzer Science Center is 1.6% +/- 0.6%lower. We extend this work to predict 24 micron flux densities for a sample of238 stars which covers a larger range of flux densities and spectral types. Wepresent a total of 348 measurements of 141 stars at 24 micron. This samplecovers a factor of ~460 in 24 micron flux density, from 8.6 mJy up to 4.0 Jy.We show that the calibration is linear over that range with respect to targetflux and background level. The calibration is based on observations made using3-second exposures; a preliminary analysis shows that the calibration factormay be 1% and 2% lower for 10- and 30-second exposures, respectively. We alsodemonstrate that the calibration is very stable: over the course of themission, repeated measurements of our routine calibrator, HD 159330, show aroot-mean-square scatter of only 0.4%. Finally, we show that the point spreadfunction (PSF) is well measured and allows us to calibrate extended sourcesaccurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of asample of nearby galaxies are identical within the uncertainties.

publications of the astronomical society of the pacific

Absolute Calibration and Characterization of the Multiband Imaging  Photometer for Spitzer. I. The Stellar Calibrator Sample and the 24 micron  Calibration