Accurate measurements of optical jitter and characterization of external disturbance sources are required to validate spacecraft system pointing jitter error budgets. In the case of the Hobble Space Telescope (HST), jitter is routinely measured using telemetry from four rate gyro assemblies sampling at 40 Hz. This sampling rate provides frequency and jitter information on low frequency oscillations due to appendage motions, but does not provide adequate characterization of frequencies and jitter of internal mechanical disturbances at higher frequencies that contribute to the total jitter error budget. Prior to launch, the HST underwent a series of optical jitter test which characterized the amount of jitter produced from various internal mechanisms which in turn were used to validate the pointing error budget. As part of the HST on-orbit system identification, under the direction of NASA/GSFC, a jitter test was performed in which one of the HST science instruments, the High Speed Photometer (HSP), was used to measure focal plane jitter during prescribed internal mechanical motion and during quiescent portions of an orbit. The objective of the HST HSP optical jitter test was to provide pitch and yaw frequency and jitter system information at a high sampled data rate measured at the HST focal plane. The HSP was selected as the measurement sensor since it is capable of sampling information at 1000 Hz. The HSP measures optical jitter by placing a bright star image on the edge of the aperture and measuring the resulting flux as the star moves in and out of the aperture. Four test were performed which each consisted of four segments that included periods where no mechanisms were operating, a period where tape recorders were commanded on, a period where the High Resolution Spectrograph was commanded on, and a second period where no mechanisms were operating. Optical jitter and frequency responses are determined by transforming the HSP flux measurements from each test segment to standard measurement values of milli-arcseconds and performing spectral analyses. Spectral analysis results are compared to ground based modal test results to determine the accuracy of on-orbit models currently in use. Jitter measurement results are calculated over one and 60 second intervals and compared to spacecraft jitter allocations. The paper concludes with recommendations on further dynamic tests to characterize spacecraft on-orbit.
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