Absolute Calibration and Characterization of the Multiband Imaging Photometer for<i>Spitzer</i>. II. 70 μm Imaging

Abstract

The absolute calibration and characterization of the Multiband Imaging\nPhotometer for Spitzer (MIPS) 70 micron coarse- and fine-scale imaging modes\nare presented based on over 2.5 years of observations. Accurate photometry\n(especially for faint sources) requires two simple processing steps beyond the\nstandard data reduction to remove long-term detector transients. Point spread\nfunction (PSF) fitting photometry is found to give more accurate flux densities\nthan aperture photometry. Based on the PSF fitting photometry, the calibration\nfactor shows no strong trend with flux density, background, spectral type,\nexposure time, or time since anneals. The coarse-scale calibration sample\nincludes observations of stars with flux densities from 22 mJy to 17 Jy, on\nbackgrounds from 4 to 26 MJy sr^-1, and with spectral types from B to M. The\ncoarse-scale calibration is 702 +/- 35 MJy sr^-1 MIPS70^-1 (5% uncertainty) and\nis based on measurements of 66 stars. The instrumental units of the MIPS 70\nmicron coarse- and fine-scale imaging modes are called MIPS70 and MIPS70F,\nrespectively. The photometric repeatability is calculated to be 4.5% from two\nstars measured during every MIPS campaign and includes variations on all time\nscales probed. The preliminary fine-scale calibration factor is 2894 +/- 294\nMJy sr^-1 MIPS70F^-1 (10% uncertainty) based on 10 stars. The uncertainty in\nthe coarse- and fine-scale calibration factors are dominated by the 4.5%\nphotometric repeatability and the small sample size, respectively. The 5-sigma,\n500 s sensitivity of the coarse-scale observations is 6-8 mJy. This work shows\nthat the MIPS 70 micron array produces accurate, well calibrated photometry and\nvalidates the MIPS 70 micron operating strategy, especially the use of frequent\nstimulator flashes to track the changing responsivities of the Ge:Ga detectors.\n