Optics for the Low and Medium Resolution Cameras

The Visible Imaging System (VIS) was designed with a stringent set of specifications in order to image the dim nightside auroras with the bright dayside of Earth within the instrument's field-of-view. The VIS is an f/8 re-imaging system with an intermediate primary focal plane and final focal planes at the sensors. The camera employs off-axis parabolic and flat mirrors that provide the image at either the low-resolution or medium-resolution sensors.

Exploded views of the VIS in the low-resolution and medium-resolution configurations are shown in Figures 2 and 3, respectively. Light enters the collimator assembly and is reflected by flat mirror M1 which is mounted on a bi-axial targeting mechanism. The targeting mechanism is driven by two motors. Operation of this bi-axial targeting mirror assembly permits the acquisition of 5.4° × 6.3° or 2.8° × 3.3° images at any position within the 20° × 20° field-of-view of the collimator for the low-resolution and medium-resolution cameras, respectively. An aperture stop is placed between M1 and the offaxis parabolic reflector M2. The aperture stop is also the entrance pupil with a diameter of 2.0 cm. M2 provides the primary image at the position of the field stop wheel. The selection of field stops on the wheel allows imaging near Earth's terminator, i.e., rejection of those portions of the primary image with sunlit Earth from entering the secondary optics. In the secondary optics the light is reflected by an angle of ~41° by the off-axis parabolic surface of M3 and becomes nearly collimated. Sensor select mirror M4 is used to direct the light into the optical path for either the low-resolution or medium-resolution sensors.

For the low-resolution sensor the nearly collimated light passes through a Lyot stop and subsequently through one of twelve filters in the filter wheel. Off-axis parabolic reflector M5 and the plane turning-mirror M6 present the focused image at the low-resolution sensor (see Figure 2). For the medium-resolution sensor the nearly collimated light from M4 passes through another Lyot stop and then through the filter wheel (see Figure 3). Off-axis parabolic reflector M7 and inverted parabolic reflector M8 provide the image at the medium-resolution sensor. The inverted parabolic reflector M8 magnifies the image by a factor of two relative to the image at the low-resolution sensor. The optical properties of each of the eight mirrors are summarized in Table 2.

The resolution of the optics for the low-resolution and medium-resolution cameras is summarized by the `spot' diagrams in Figure 4. These spot diagrams are given at the position of the respective sensor input planes and for the four corners and center of the respective fields-of-view. For comparison, the linear dimensions of two adjacent pixels at the sensor input image plane for the 256 × 256-pixel format are shown. The modulation transfer functions (MTFs) of the optics, exclusive of the sensor, are given in Table 3 for the low- and medium-resolution cameras. The diffraction limit is also specified.

The mirrors were manufactured by Speedring Systems, Inc. of Rochester Hills, Michigan. With the exception of M1 which is Zerodur, each mirror is a beryllium substrate that is diamond turned, coated with electroless nickel, and subsequently lapped to final figure with a low-scatter super-polish with surface roughness of 1 nm. Vapor-deposited aluminum is used to increase the reflectivity to greater than 90%, and an overcoat of SiO₂ is applied to protect the surface from degradation. The mirrors are mounted with beryllium brackets to a beryllium optical bench to achieve an athermal design. Thermal analysis shows that temperature differences should be less than 10° C and will not significantly affect the alignment of the optical elements.