Overview of Instrumentation

A photograph of the instrumentation is shown in Plate 1. The instrument is to be mounted on the despun platform of the Polar spacecraft. The top covers are removed. These covers are radiators equipped with optical surface reflectors (OSRs) for passively cooling the chargecoupled devices (CCDs) and the electronics. A diagram of the instrument as viewed from this top side is shown in Figure 1 and is useful in identifying the various subsystems that are visible in Plate 1. Two major compartments are visible in Plate 1, the optics section on the lefthand side and the electronics section on the right. The gold-colored aperture door and two rectangular collimators extend forward of the main optics section. The motor-driven door provides protection for the internal optics against particulate material and condensables during launch and inflight reorientations of the spacecraft spin axis. The instrument is assembled in a Class-100 clean room. The rectangular collimator on the lefthand side services the two cameras for auroras at visible wavelengths. The field-of-view provided by this collimator is 20° × 20°. These two cameras share primary optics and some of the secondary optics. The cameras are nearly identical with the major exception of angular resolution. The angular resolution of the medium-resolution camera is 0.011° × 0.013° (pixel size) and that of the low-resolution camera is 0.022° × 0.025°. The instantaneous fields-of-view of these cameras is significantly less than that provided by the collimator. Thus a bi-axially rotated mirror is employed to cover this entire field-of-view by mosaicing images, particularly at low altitudes. An overview of the performance parameters for the cameras is given in Table 1. At the primary focal plane a fieldstop wheel is used to block the image of sunlit Earth from the secondary optics. This field stop wheel thusly prevents this intense light from direct entry into the secondary optics. The optical path is then folded in a complex geometry to accommodate the allowed dimensions of the housing. A plane mirror rotated by a motor is used to determine which of the two cameras receives the image. The light is collimated and passed through a selected narrow-band filter (the gold-colored wheel) and the image is reformed at the faceplate of an image intensifier. The intensified image is then optically transferred to a CCD. This mechanical isolation allows the cooling of the CCD to temperatures in the range of -90° C in order to obviate the deleterious effects of damage from energetic ions in the inner radiation zone. At these temperatures, the electrons associated with the displacement defects are trapped. The small gold-colored ``knobs'' provide the heat strap connection to the topside radiator.

The smaller rectangular collimator behind the right-hand side of the door is used for the Earth camera. This Earth camera provides a 20° × 20° image without the need for mosaicing several images. The angular resolution is lesser than that of the visible cameras, 0.08° × 0.08°. For comparison the corresponding pixel for the imagers on DE-1 is circular with angular diameter 0.25°. The Earth camera is equipped with one broad-band filter at far-ultraviolet wavelengths. These images are processed within the VIS to ascertain that no intense light sources, such as sunlit Earth, are viewed by the secondary optics of the visible cameras. It is possible to telemeter one image from the Earth camera each 12 s.

The electronics compartment is on the righthand side of the instrument as viewed in Plate 1. The electronics stack in the rear of this compartment is three identical sets of power supplies and control electronics for the three sensors. The front stack includes the six microprocessors with a total of 736 kbytes of memory for operating the instrument and for data compression. The two primary power convertors are located out-of-sight below the sensor electronics.