Each detector contains its own internal 55Fe calibration source, which illuminates an area of the detector outside the field of view with 5.9 keV X-rays. These data will be used to monitor gas gain and energy response. In addition, when not in the telescope focus, the full active areas of the detectors can be illuminated with X-rays from a number of 55Fe sources through a multiple collimator. This produces 62 point-like spots within the central 150 mm of HEPC and 33 point-like spots within the central 70 mm of LEPC. From these data, detector spatial response, energy response, efficiency and background rejection efficiency, and spatial gain variations can be determined. Observers can request out-of-focus calibration exposures before and after their observations.