PRESERVICING MISSION, ON-ORBIT MODIFICATIONS TO HUBBLE-SPACE-TELESCOPE POINTING CONTROL-SYSTEM

Because of unexpected, thermally induced disturbances originating in t he solar arrays (SAs), the Hubble Space Telescope was initially unable to meet its pointing error specifications of 0.007 arc-s [root mean s quare] for observations lasting from a few seconds to several hours. T he effort to pinpoint the mechanism causing the problem and to redesig n the onboard controller to accommodate the disturbance began shortly after deployment. After controller modifications, the pointing errors were well within the specification for all of the orbit except for sho rt intervals during transitions in and out of Earth's shadow. This pap er presents a chronology of the redesign process and night data showin g the achieved pointing performance. During the recent servicing missi on (December 1993) the SAs were replaced with arrays of a modified mec hanical design. A preliminary assessment of pointing performance since that time is presented.