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Chapter 7. Mechanical Design
The LASCO experiment consists of two boxes, the Coronagraph Optical Box (COB) and the LASCO Electronics Box (LEB). The COB contains the three coronagraph optical systems, C1, C2 and C3, each with its optical axis pointed at Sun center. The COB has dimensions of 135 x 34 x 32 cm. Optical axes are parallel to the long axis of the box, and to the solar vector. View apertures for each telescope are at the sunward end of the box. The COB is separable into two sections, illustrated in Figure 7-1. One section contains C1, and the other both C2 and C3. Each section is milled from heavy aluminum 6082-T651 plate, selected for its low stress corrosion and for dimensional stability, and forms the structure upon which lens mounts, baffles, filter wheels, etc. are mounted. Each section is milled with a system of ribs, bulkheads and mounting pads, with thin walls between. When assembled, the interlocking edges, plus ribs and bulkheads, form a trussed structure for maximum stiffness. Lens mounts and baffles are of aluminum alloy (e.g. 6061-T6), and blackened with materials of low vapor pressure: either chemically blackened, black anodized (vacuum baked to drive off condensables), or black teflon coated.
The COB experiment layout allowed three co-aligned instruments to be assembled, aligned, and tested at three separate consortium institutions. After assembly, co-alignment was checked and corrected by minor adjustments, such as adjustment of external occulting disks, or internal adjustments reached through small access ports. The design allowed the three systems to be thermally coupled, so that all would operate in the same temperature range.
The COB is isostatically mounted using three stress-relieving legs, an adjustable bipod in back and two non-adjustable monopod and tripod legs in the front. The LASCO Electronics Box (LEB) is hard mounted to the spacecraft. Isostatic mounting, with position adjustments by the rear leg, permits the COB to achieve the required pointing accuracy and stability. The mounts of both the COB and LEB provide thermal isolation from the Spacecraft Payload Module.
7.1 Experiment Alignment
Once on station, the pointing of the COB will be determined by the C2 optical system, to maximize the stray light rejection in the C2 optics. Each of the three optical systems (C1, C2, and C3) have their own optical axes. Prior to delivery of LASCO, boresighters were co-aligned with auto-collimating telescopes to align the boxes to each other to within 15 arc sec, and to the reference axis. Shims were used to adjust the COB alignment at mounting to the spacecraft to within 3 arc min. Launch and thermal distortions may add up to 5 arc min to the misalignment between the spacecraft and the LASCO pointing axes. To compensate for this misalignment, the rear leg actuators can move the entire COB to Sun center in steps of 2 arc sec. The total range of motion is greater than 15 arc min.
7.2 M1 Mirror Mount
The M1 mirror mount can perform small angular movements of the M1 objective mirror of the C1 telescope, to compensate for spacecraft pointing errors or other flight-induced misalignments. It will also be used to increase the resolution of C1 through dynamic imaging (described in Chapter 4). The M1 mirror is held by a three-point mount. Orthogonal tip, tilt, and fore-and-aft motions are provided by low voltage piezo-electric transducers. This will be used occasionally for pointing corrections, once after launch to bring the C1 instrument into optimal alignment, and later when thermal or other changes occur.
7.3 Aperture Doors
Each of the three telescopes in the COB has its own front aperture door, to seal the instrument against contamination during storage and launch, and during spacecraft station-keeping maneuvers. The door is moved by a stepper motor driving a lead screw inside the door hinge. All bearings are of low abrasion type, to avoid generation of contaminating particles. The door mechanism is designed to open and close the door at least 1000 times.
Since the opening mechanism obviously carries the risk of a single point failure, a redundant one-time opening mechanism is provided. With this, the door arms are released from the lead screw by a paraffin actuator on ground command, and the door is permanently rotated back by means of a helical torsion spring. In addition, the LEB contains redundant motor driver circuits for the COB opening mechanism.
7.4 Filter/Polarizer Wheels and Shutters
Each of the three telescopes has a filter wheel, a polarizer wheel, and a shutter. Both wheel diameters are 10 cm, and they are driven directly by 1.8 degree stepper motors. The mechanical shutter is a blade shutter with a 90 degree stepper motor. The shutter blade is indexed in or out of the light path by the motor. Redundant motor driver circuits are provided by the LEB.
The shutter must be opened and closed for every exposure. About 120 cycles a day are planned for the C1 and C2 telescopes. The filter wheels will be indexed less frequently, about 50 times per day for C1, and 25 per day for C2 and C3. The filter wheels are located in an area of the COB less sensitive to particulate contamination, but sealed motors are still used.
7.5 Internal Occulter (D2) Centering System
The internal occulter D2 of the C2 telescope is movable in the plane perpendicular to the optical axis, to compensate for small misalignments. Small sealed stepper motors with eccentric shafts provide travel in the micron range. Redundant motor driver circuits are provided by the LEB. This mechanism is used to compensate for small but very important misalignments that may occur during launch, or as a result of thermal distortions.
7.6 Focus Mechanisms
The C1 and C2 telescopes each include a focus mechanism. A lens mount is moved along the optical axis by a mechanical linkage to a stepper motor. This motor is the same type described for the previous mechanisms. Redundant motor driver circuits are provided by the LEB. The focus mechanisms will be used infrequently, to compensate for changes in the thermal equilibrium which may defocus the optics. The depth of focus of the C3 telescope is greater than the defocussing that would occur for any realistic temperature departure from the nominal 20 C.