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Published September 2007 | Published + Accepted Version
Journal Article Open

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 μm 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 μm channels to the 160 μm 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 μm flux density is predicted from those fits and compared with the observed 160 μm 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 μm 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 μm 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.

Additional Information

© 2007 Astronomical Society of the Pacific. Received 2006 November 15; accepted 2007 July 25; published 2007 September 28. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. Support for this work was provided by NASA through contract 1255094 issued by JPL/Caltech. Ephemerides were computed using the services provided by the Solar System Dynamics group at JPL. We thank an anonymous reviewer for input that improved this paper significantly. And we acknowledge the wise insight of Douglas Adams, who pointed out over 20 years ago that the answer is 42.

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Published - STApasp07.pdf

Accepted Version - 0707.2103.pdf

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Created:
August 19, 2023
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October 19, 2023