Absolute calibration and characterization of the multiband imaging photometer for Spitzer. II. 70 μm imaging

The absolute calibration and characterization of the Multiband Imaging Photometer for Spitzer (MIPS) 70 μm coarse- and fine-scale imaging modes are presented based on over 2.5 yr of observations. Accurate photometry (especially for faint sources) requires two simple processing steps beyond the standard data reduction to remove long-term detector transients. Point-spread function (PSF) fitting photometry is found to give more accurate flux densities than aperture photometry. Based on the PSF fitting photometry, the calibration factor shows no strong trend with flux density, background, spectral type, exposure time, or time since anneals. The coarse-scale calibration sample includes observations of stars with flux densities from 22 mJy to 17 Jy, on backgrounds from 4 to 26 MJy sr ^-1, and with spectral types from B to M. The coarse-scale calibration is 702 ± 35 MJy sr ^-1 MIPS70 ^-1 (5% uncertainty) and is based on measurements of 66 stars. The instrumental units of the MIPS 70 μm coarse- and fine-scale imaging modes are called MIPS70 and MIPS70F, respectively. The photometric repeatability is calculated to be 4.5% from two stars measured during every MIPS campaign and includes variations on all timescales probed. The preliminary fine-scale calibration factor is 2894 ± 294 MJy sr ^-1 MIPS70F ^-1 (10% uncertainty) based on 10 stars. The uncertainties in the coarse- and finescale calibration factors are dominated by the 4.5% photometric repeatability and the small sample size, respectively. The 5 σ, 500 s sensitivity of the coarse-scale observations is 6-8 mJy. This work shows that the MIPS 70 μm array produces accurate, well-calibrated photometry and validates the MIPS 70 μm operating strategy, especially the use of frequent stimulator flashes to track the changing responsivities of the Ge:Ga detectors.