The mirror/occulter mechanism (MOM) consists of independent mirror and internal occulter mechanisms that are jointly controlled and coordinated by the instrument flight software. The three mirrors image segments of the corona on their respective spectrometer entrance slit. The mirror mechanism consists of a mirror/carrier subassembly which supports two ultraviolet mirrors and one coated visible light mirror. These mirrors are aligned and mounted (via lapping and shimming) onto the carrier such that their foci are coaligned to within thirty arcseconds. The mirror/carrier subassembly is supported by flex pivots, actuated by a voice coil actuator (VCA) and has its position sensed by differential impedance transducers (DIT). The VCA and DIT subsystems provide mirror position control, readout precision and repeatability to within 2.5 arcseconds and maintain physically-redundant actuating and sensing elements.
The internal occulter mechanism blocks the portion of each telescope mirror that would otherwise specularly reflect diffracted light from the external occulter into the respective spectrometer entrance slit. The mechanism consists of an occulter plate subassembly supported on each side by thin steel bands which are integrated within a rotational drive system. This drive system consists of a drive pulley controlled by a limited-angle torquer motor and sensed by a resolver and an idler pulley subassembly that converts a 57 drive pulley rotation into the 36 mm occulter translational range required to occult each mirror for observations from 1.2 to 10 . The torquer motor/resolver subsystems are physically-redundant and provide internal occulter edge positional control to less than 0.1 mm. The optical characteristics of the internal occulter are discussed in Section 6.
The MOM also maintains independent launch lock mechanisms which utilize paraffin actuators to constrain the mirror/carrier and occulter plate subassemblies during launch and prevent unacceptable mechanical loads from being transmitted through nonstructural elements of the subassemblies. The mirror/carrier and occulter plate launch locks are positioned to place the mirror/occulter at its approximate 2.7 position. These actuators then provide the ability to release these launch constraints via instrument command to allow on-orbit mechanism function. The MOM is aligned and integrated within the telescope assembly using a kinematic mount support system of three titanium uniaxis mounts and one three-axis mount assembly. This mount system allows global motions of the MOM during its alignment to the telescope and also isolates differential structural deformations between the interfacing telescope assembly and MOM housing structures.