Mechanical Design

The VIS mechanical structure consists of three major subassemblies; the optics housing, the electronics housing, and the radiator. In addition the various motors that are employed in the instrument are described in this section. The optics housing is designed to provide a stiff, stable optics platform for the two visible cameras and the Earth camera. This housing is required to be light-tight and to be of minimum mass. The optics housing assembly includes the optical bench, optical subassemblies, and the collimator extension subassemblies. The optical subassemblies are mounted to a pocketed beryllium base plate that serves as the optical bench. The structure is optimized for the reduction of thermal gradients.

The optical bench is enclosed by light-tight, pocketed Mg walls. Each interface, i.e., optics wall-to-bench and optics wall-to-wall, is baffled and sealed with O-rings. To prevent the possibility of distortion of the optical bench by the difference in the thermal expansion coefficients of Be and Mg, attach points are designed to eliminate fastener interference.

The collimator extensions include the Earth-camera collimator, the collimator for the two visible cameras, and an aperture door for the collimators. Each collimator is machined from Mg stock and subsequently etched to minimize mass. The baffles in the collimators are coated with Martin Black. Each of the two collimators is thermally isolated from the optics housing. A special light-tight seal is used to prevent entry of stray light into the optics housing, without metal-to-metal contact at this interface. The aperture door is machined from a thick Mg plate to produce a stiff, low-mass structure.

The electronics housing is fabricated from Mg and is attached to the optics housing by a light-tight interface wall which also accommodates the electrical interface between the optics and electronics sections. Because this housing interfaces with the optics housing, the radiator, and the spacecraft platform, careful consideration is given to the structural dynamics of these interfaces in order to prevent distortion of the optical bench.

The instrument thermal radiator consists of two thermally isolated plates that are covered with OSRs. One of these radiators is used to dispose of the thermal energy from the power dissipation in the optics and electronics sections and to maintain the temperature of the optical bench in the range -20° to 0° C. The larger radiator is used to passively cool the CCDs to 90° C. This radiator is coupled directly to the tantalum radiation shield surrounding each CCD by means of a copper heat strap.

There are six stepping motors and one wax motor in the VIS. The six motors are manufactured by Schaeffer Magnetics, Inc. and are two similar types. The first type is used for the bi-axial targeting mirror (two motors), the sensor select mirror, and the collimator door. The second design is used for the filter wheel and the field stop wheel. Both motors are similar except that the motor step for the first type is 3.75° and is reduced by an integral 80:1 harmonic drive, and the motor step for the second type is 1.5° with no harmonic drive. Both motor types operate efficiently at 100 steps/s and have the capability of bidirectional rotation. The two motors for the bi-axial targeting mirror provide motion of the fields-of-view for the visible cameras with respect to the platform in two orthogonal planes, i.e., in a plane parallel to the spacecraft spin axis and in a plane perpendicular to this axis. The increments in the field-of-view are 0.094° in either direction. The Starsys wax motor is used for a one-shot mechanism that permanently opens the collimator door in the event that the door motor is not operable.