Auroral imaging from a spinning satellite

Abstract

For optimizing in situ particle and field measurements, auroral research satellites are best operated in a spinning mode. Simultaneous imaging of the optical aurora from such satellites requires either a stable platform or the derotation of the camera itself. Either of these requirements is complex and expensive. Either of these solutions also suffers from the problem that image blur often occurs due to the misalignments between the actual and the nominal spin axes of the satellite. Here we propose a novel solution in which the camera(s) are mounted solidly on the spacecraft to observe parallel to the spin axis of the satellite while a despinning flat 45° mirror directs the field of view toward the spacecraft nadir. The resultant image will appear to rotate in the frame of reference of the detector in the camera. In our scheme the images are exposed rapidly and a derotation algorithm is applied to the coordinates of each pixel in real time before the images are co-added in memory. The derotation algorithm uses only look up tables and integer additions and can be executed rapidly in hardware so that the system can support relatively fast satellite spin cycles. The system was simulated including a 1.8° misalignment between the nominal satellite spin axis (parallel to the mirror rotation axis) and the actual spin axis. It was shown that the look up table based algorithm can despin the images and correct for the axes misalignment, allowing the observation of the aurora at full resolution and with continuous coverage.