MIPS: Accuracy, Stability, and Repeatability

Figure: Repeatability of photometry at 24 microns. Note the scale on the y-axis.

Figure: The 24 micron calibration factor as measured for the calibration star measurements (aside from two high and two low outliers) over a large dynamic range in predicted flux. Note the scale of the y-axis.

The 70 micron calibration is based (see section 8.1.3.6 in the SOM) on solar analog, A, and K giant stars. Using all the calibration star measurements taken to date, the accuracy of the 70 micron measurements is around 7%. These calibration measurements include stars with predicted fluxes ranging from 30 mJy to 5 Jy on backgrounds between 4 and 25 MJy/sr. One of the calibration stars (HD 163588; see Figure below) is measured every MIPS campaign and the repeatability of this star is better than 4%. The main difference between these two accuracies is likely the result of the nonlinearities still to be corrected (see the MIPS Data Handbook, or the Gordon et al. papers on data reduction, or Rieke et al., 2004, ApJS, in press, all available here on the MIPS documents section of the SSC website, for more details).

Figure: Ten measurements of the K2III star HD 163588 at 70 microns, distributed over seven campaigns. Note vertical scale; 70 micron array produces 5% rms repeatability.

Unlike the other two arrays, the calibration of the 160 micron data is based on a variety of objects (see section 8.1.3.6 in the SOM) including asteroids. The resulting multiple calibrations agree to within 12%. However, because these were not the planned calibration sources, the 160 micron flux calibration is currently more uncertain than for the other two MIPS arrays. Improvements in the calibration will depend largely on further observations and modelling of asteroids.

All archived MIPS data will be periodically reprocessed as significant changes in the calibration are determined and implemented. The 160 micron data offer the greatest challenge to the calibration process, and it is likely that those pipelines and data will undergo the most modifications as they continue to improve over the first several years.

Interested readers should also consult the SOM, sections 8.1.3.5 and 8.1.3.6 on calibration in general and section 8.3.1 on specific calibration procedures, in addition to the MIPS Data Handbook.

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