A far ultraviolet imager for the International Solar-Terrestrial Physics mission

The Ultraviolet Imager for the Global Geospace Sciences portion of the International Solar-Terrestrial Physics program is described. The instrument operates in the far ultraviolet (FUV) and is capable of imaging the auroral oval regardless of whether it is sunlit or in darkness. The instrument has an 8-deg circular field of view and is located on a despun platform which permits simultaneous imaging of the entire oval for at least 9 hours of every 18 hour orbit. The three-mirror unobscured aperture optical system (f/2.9) provides excellent imaging over this full field of view, yielding a per pixel angular resolution of 0.6 milliradians. Its FUV filters have been designed to allow accurate spectral separation of the features of interest, thus allowing quantitative interpretation of the images. The system has been designed to provide ten orders of magnitude blocking against longer wavelength (primarily visible) scattered sunlight, thus allowing the first imaging of key spectrally resolved FUV diagnostic features in the fully sunlit midday aurorae. (AIAA)