The roll and pointing mechanisms allow UVCS to scan heliocentric heights from 1.2 to 12 R and roll the UVCS FOV about Sun-center in order to observe any position angle, and to correct for any SOHO Sun-center misalignment. The roll mechanism, located at the aft end of the instrument, can rotate the telescope assembly over a 179.75 range. Rotation is induced by a stepper motor driving a gearhead/harmonic drive which provides an approximate 5000:1 gearing ratio from the stepper motor. Electrical connections between the REU and interior telescope hardware are routed through a flexible lead assembly which maintains a wire-for-wire conductance over the full rotational range. The baseline rotation rate is 8 per minute and variable rates between 3 and 15 per minute are available for implementation through the flight software. The telescope rotational position is sensed by a resolver which allows rotational position control to 0.1.
The offset pointing mechanism allows an offset pitch and yaw pointing of the telescope assembly over a 54 arcminute range. The telescope roll ring subassembly possesses an integral roll/pointing ring which rests upon two crowned and hardened front support rollers at symmetric positions about the telescope centerline. The rollers are supported by roller bearings mounted by metallic stands which are driven inward and outward by translation stages, providing a bidirectional motion of the telescope assembly. Retention springs integral within the roller bearing subassemblies restrain the telescope roll/pointing ring onto and maintain constant contact between the ring and the drive rollers. Offset pointing positions within 16 arcminutes of Sun-center are measured by the Sun sensor (see Section 4.3) while pointing positions outside of this central zone are computed based on position telemetry of the two translation stages. The fine-pointing Sun sensor, which consists of four redundant photodiodes located at positions about the Sun-center reference line, provides a Sun-center determination to within 4 arcseconds by measuring relative photodiode signal intensities. This fine pointing control can be utilized following roll maneuvers to eliminate any Sun-center misalignment caused by the telescope roll mechanism. The translation stage drive capacity allows offset pointing at a baseline rate of approximately one arcminute per minute, but slower and faster pointing rates are available for implementation through the flight software.