Stellar calibration of the Special Sensor Ultraviolet Limb Imager (SSULI) on the DMSP spacecraft

Abstract

The SSULI (Special Sensor Ultraviolet Limb Imager) is a limb-scanning far- and extreme-ultraviolet spectrometer flying on the Defense Meteorological Satellite Program (DMSP). The sensor sensitivity is tracked through the mission life by taking advantage of serendipitous stellar apparitions which, over the course of several days, track across the sensor’s field of view, allowing for not only the production of a sensitivity curve when compared against the known stellar spectra, but additionally pointing information and field-of-view information can be gleaned from comparing the star’s expected and observed positions. Most notably, because the star’s apparition traces across the field of view predictably in one axis, and randomly in another, multiple apparitions of these point sources can be used to map out the gain on the detector’s entire surface, revealing the existence and extent of localized gain sags. Additionally, multiple, routinized, and scheduled calibrations can be used to track sensor behavior through the mission life, including effects like a detector scrub, photocathode performance, and possible optics contamination. Results from the SSULI 002/DMSP F18 and SSULI 005/DMSP F19 stellar calibrations are presented.