MIPS : Data Calibration

The absolute calibration of MIPS data proceeds in three steps. First, we have determined the best possible calibration of our flux standards at 10.6 microns using other instrumentation. Second, we use "standard" stellar spectral energy distributions normalized to 10.6 microns to extrapolate from that wavelength to the MIPS bands. Additional calibration tasks using asteroids were developed at 160 micron to cope with the ~1.2 micron light leak. Finally, for the 70 and 160 micron germanium arrays, stimulator flashes provide continuous tracking of responsivity changes since the last observation of a calibration source. Calibration sources are observed at the beginning and end of each MIPS observing campaign to allow the removal of any second-order changes in the calibration of the germanium detectors.

Except for the occasional dedicated calibration AOR of celestial standards, an individual science AOR does not depend on any other science AORs for obtaining calibration. Each MIPS AOR is internally calibrated. Because of the use of the stimulators as relative calibration sources, this method is robust and stable over an instrument campaign. This relative calibration method also allows the varying instrument response to be frequently referred to the signals from the celestial standards. For the Ge:Ga arrays, the stimulators are flashed approximately every 2 minutes or less as an integral part of the basic observation sequences. Because of the inherent longer term stability of the Si:As array, that detector uses much less frequent stimulator flashes. The frequency of stimulator flashes is determined by the SSC and implemented in the AOT design, and cannot be selected by the individual observer.

Detector dark current measurements are made for each array individually by positioning the scan mirror such that the array views only the interior of the cold instrument. The optical layout of MIPS is such that only one array at a time can be completely hidden from light entering through the telescope. In fact, only about two thirds of the 70 micron array can be placed in the dark at once, so for that array the dark current measurement is made in two pieces. Dark current measurements are expected to only be required at the beginning and end of each MIPS campaign. To avoid any scattered light from nearby very bright sources affecting the measurement, the telescope is pointed toward a region known to be free of such sources during the dark current measurement sequence.

Flatfielding of the Ge:Ga detector arrays is achieved by using calibrated stimulator illumination patterns. The stimulators are calibrated using a flat field matrix determined from "uniform" sky measurements at the start and end of instrument campaigns. The flatfield procedure takes advantage of fundamental MIPS observation techniques to obtain the highest quality flatfield data possible (median combine of multiple scan map legs with source rejection, for example). Standard astronomical observation practices are used in determining the flatfield response of the arrays and in the processing of the data.

For more information, see the MIPS chapter of the SOM

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