Characterization of MIPS 70μm flux non-linearity

This document describes the work undertaken for characterizing the flux non-linearity of the MIPS 70μm array. The effect, due to the change of rate of incident radiation on the detector, has been described in Gordon et al. (2007) and is typical of Ge:Ga photoconductors. The analysis has made use of MIPS calibration data aquired in Wide Field (WF), Narrow Field (NF) and Spectral Energy Distribution (SED) mode, and has attempted the characterization of this effect in terms of flux density as well as of surface brightness. Indications of a departure from a linear behaviour are present in both cases. The analysis shows that the NF measurements are less severely affected by a non-linear behaviour with respect to WF observations. In particular, the MIPS 70μm NF data of targets brighter than ∼ 2 Jy underestimate the true flux up to 20%, while WF measurements underestimate the true flux up to 50%. A functional form characterizing the non-linear behaviour of the NF and WF measurements is provided. We have also rederived the calibration factor for NF, before and after applying the non-linearity correction described above. For this purpose we make use of a data set of calibration stars which nearly doubles the sample in Gordon et al. (2007). The calibration factor obtained before correcting for non-linearity effects is consistent with Gordon et al. (2007). However, when non-linearity effects are taken into account, we show that the calibration factor is lower by ∼ 20%.