Guiding Errors in 3-Gyro: Experience from WF/PC, WFPC2, STIS, NICMOS and ACS

As background information to support interpretation of results during testing under 2-gyro operation, this report summarizes typical behavior gleaned from several observations with ACS, WF/PC, WFPC2, STIS and NICMOS. The observations used have all been for at least five consecutive orbits, and have collected data with the goal of producing very high signal-to-noise, time-series photometry for which definition of x, y offsets in time, and characterization of plate scale changes are an inherent part of the science analysis. In 3-gyro guiding mode it is common to see changes within single orbit visibilities at a level over 10 mas. During the first orbit, and transition to the second, pointing errors are slightly larger than those seen in subsequent orbits. Experience from a total of 385 prime plus parallel orbits is detailed here to give a broad overview of HST guiding accuracy.