Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer . III. An Asteroid‐based Calibration of MIPS at 160 μm

Abstract

We describe the absolute calibration of the Multiband Imaging Photometer for Spitzer (MIPS) 160 mum channel. After the on-orbit discovery of a near-IR ghost image that dominates the signal for sources hotter than about 2000 K, we adopted a strategy utilizing asteroids to transfer the absolute calibrations of the MIPS 24 and 70 mum channels to the 160 mum channel. Near-simultaneous observations at all three wavelengths are taken, and photometry at the two shorter wavelengths is fit using the standard thermal model. The 160 mum flux density is predicted from those fits and compared with the observed 160 mum signal to derive the conversion from instrumental units to surface brightness. The calibration factor we derive is 41.7 MJy sr-1 MIPS160-1 (MIPS160 being the instrumental units). The scatter in the individual measurements of the calibration factor, as well as an assessment of the external uncertainties inherent in the calibration, lead us to adopt an uncertainty of 5.0 MJy sr-1 MIPS160-1 (12%) for the absolute uncertainty on the 160 mum flux density of a particular source as determined from a single measurement. For sources brighter than about 2 Jy, nonlinearity in the response of the 160 mum detectors produces an underestimate of the flux density: for objects as bright as 4 Jy, measured flux densities are likely to be ~=20% too low. This calibration has been checked against that of the ISO (using ULIRGs) and IRAS (using IRAS-derived diameters), and is consistent with those at the 5% level.