Photon-by-photon post-processing correction of pointing errors in an orbiting satellite

Abstract

The Jupiter Magnetosphere Explorer (JMEX) is a proposed earth-orbiting satellite which will image the planet Jupiter in the FUV with a 0.5 m telescope at 0.25 arcsec (") resolution. Because the satellite is small and lightweight, vibrations from the reaction wheels (even though isolated by dampers) produce random pointing errors with an amplitude as large as 5" at a frequency around 1 Hz. In order for the telescope to achieve a resolution of 0.25" FWHM during long exposures, we will use a novel post-processing scheme to correct the pointing error. The UV science camera is a photon-counting MCP detector which produces data as a time-stamped photon list with 0.08" spatial resolution and roughly 1 ms temporal resolution. Simultaneously, a 0.5" pixel video camera, fed by a pickoff mirror in the main beam, captures visible images of the planet's disk at 30Hz and, with onboard processing, the centroid of the planet is determined, frame-by-frame, with a resolution <0.02" (1/25 pixel). With inter-frame interpolation, each photon from the UV camera is position-corrected in ground post-processing to an accuracy of 0.02".To rigorously test this scheme, we have constructed a hardware mock-up consisting of a tip-tilt mirror, a beamsplitter, and two video cameras with controlled noise characteristics. The tip-tilt mirror produces controlled image motion over a range of amplitudes and frequencies. With all parameters at worst-case values, we have verified the specified performance of the system and achieved centroid correction close to the limit set by counting noise statistics.