Description of the Spectrographic Imager (SI)

The Spectrographic Imager is designed to image the whole Earth proton aurora from satellite distances greater than 4 Earth radii to the center of the Earth. It uses a reverse Wadsworth design to select the Doppler shifted Lyman H-alpha line at 121.82 nm in the ultraviolet part of the optical spectrum and to reject the non-Doppler shifted Lyman H-alpha from the geocorona at 121.567 nm. The field of view is 15x15 degrees. The temporal resolution between two images is 120 s and the size of the final images is 128x128 pixel elements, corresponding to spatial resolutions of less than 100 km at apogee distances.

Doppler shifted spectral line at 121.82 nm
Field of view 15x15 degrees
Resolution elements of less than 0.15 degrees
Temporal resolution 120 s
Goal sensitivity 100 R in final image in presence of 10 kR at 121.6 nm

The output of the Spectrographic Imager was simulated with a model of the auroral oval and different count rates in individual pixels from one to ten counts per frame.

Last update: 27 June 1997