Absolute Calibration and Characterization of the Multiband Imaging Photometer for<i>Spitzer</i>. I. The Stellar Calibrator Sample and the 24 μm Calibration

Abstract

We present the stellar calibrator sample and the conversion from instrumental\nto physical units for the 24 micron channel of the Multiband Imaging Photometer\nfor Spitzer (MIPS). The primary calibrators are A stars, and the calibration\nfactor based on those stars is 4.54*10^{-2} MJy sr^{-1} (DN/s)^{-1}, with a\nnominal uncertainty of 2%. We discuss the data-reduction procedures required to\nattain this accuracy; without these procdures, the calibration factor obtained\nusing the automated pipeline at the Spitzer Science Center is 1.6% +/- 0.6%\nlower. We extend this work to predict 24 micron flux densities for a sample of\n238 stars which covers a larger range of flux densities and spectral types. We\npresent a total of 348 measurements of 141 stars at 24 micron. This sample\ncovers a factor of ~460 in 24 micron flux density, from 8.6 mJy up to 4.0 Jy.\nWe show that the calibration is linear over that range with respect to target\nflux and background level. The calibration is based on observations made using\n3-second exposures; a preliminary analysis shows that the calibration factor\nmay be 1% and 2% lower for 10- and 30-second exposures, respectively. We also\ndemonstrate that the calibration is very stable: over the course of the\nmission, repeated measurements of our routine calibrator, HD 159330, show a\nroot-mean-square scatter of only 0.4%. Finally, we show that the point spread\nfunction (PSF) is well measured and allows us to calibrate extended sources\naccurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of a\nsample of nearby galaxies are identical within the uncertainties.\n