Limitations on Target Viewing Intervals for the Hubble Space Telescope Sensitivity to Orbit Prediction Errors and Avoidance of the Bright Earth and Sky Background -i

Sensitivity studies of target viewing interval duration to orbit prediction error performed with a numerical simulator of the ground system reveal high prediction accuracy over current one-year and even two-year planning intervals. With the approach to solar maximum in the late 1990's, sensitivity to orbit plane and altitude errors increases slightly. Typical errors are less than 1% for sub-orbital visibility durations and less than 5% for multi-orbital durations. Revision of long range plans is recommended twice a year at solar maximum. Recent reduction of the bright and dark earth clearance constraints for the Fine Guidance Sensor system have increased visibility duration for guide star acquisition and maintenance. The next step is to reduce the instrument specific constraints when amenable to increase time on target. Observations in the U-band, where earth albedo and zodiacal light intensity are low, is a prime candidate. Only the presence of emission lines from the atmosphere (principally Lyman-a and oxygen-I spectral lines) imposes a limit for observations at the applicable wavelengths. Estimates of maximum possible gain for the reduction of instrument constraints when permitted by low levels of background show a benefit both for sub-orbital viewing durations and multiorbital viewing durations. Visibility interval duration sensitivity to the bright earth and sky background is complicated by dependence on target magnitude, nearby sources, wavelength, instrument throughput, and observation timeof-year and duration. The background light model, consisting of bright earth, zodiacal light, and geocoronal emission, is calibrated at low off-axis angle against near-instantaneous data from the Fine Guidance Sensors. Comparison is made with data from the Wide Field Planetary Camera. High Speed Photometer data are shown which reveal the extent of the bright-dark earth terminator.