We select certain pointings to provide flat fields for all 3 cameras; see Figure 4.1. These fields are known to have bright, relatively unstructured Zodiacal light, no bright stars or galaxies, and limited (uncrowded) faint sources. The same positions are used for all three bands. Since the saturation levels at 70 microns are higher than expected pre-launch, we use bright Galactic cirrus rather than Zodiacal light for MIPS SED illumination corrections.
Flat-fielding 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 flat field procedure takes advantage of fundamental MIPS observation techniques to obtain the highest quality flat field data possible (median combine of multiple scan map legs with source rejection, for example). Standard astronomical observation practices are used in determining the flat field response of the arrays and in the processing of the data.
Figure 4.1: MIPS flat field positions indicated on a DIRBE 140 micron all-sky map.