Stray Light Rejection

An important consideration in the design of the VIS is the adequate rejection of stray light. At visible wavelengths the sunlit Earth can be brighter by a factor of 10⁶ than the dim nightside auroral emissions. One of the most important features of the VIS for the rejection of stray light is the lowscatter superpolished mirrors. The values for their bidirectional reflectance distribution function (BRDF) are 10², 3 × 10⁴, 6 × 10⁵, and 8 × 10⁶/sr for angles of 1°, 3.5°, 10° and 45° from the specular direction, respectively. The corresponding BRDF values for the Zerodur M1 mirror are 5 × 10⁴, 2 × 10⁵, 1 × 10⁵ and 3 × 10⁶/sr, respectively. A further important aspect of the design is the use of offaxis optical elements to ensure that no mechanical structure intrudes into the primary or secondary optical paths. Such intrusion can cause sufficient scattering to overwhelm the intensities of nightside auroral emissions.

There are three critical stops in the optics. The aperture stop rejects all rays from outside the fields-of-view at the sensors, e.g., from the collimator baffle edges and M1 mirror mount. Secondly a Lyot stop is located between mirror M4 and the filter at the position corresponding to the image of the aperture stop. This Lyot stop rejects stray light scattered from the internal baffles between the aperture stop and itself and from the edges of the aperture stop. The third critical stop is the field stop at the primary image plane. For images taken when viewing near the terminator an appropriately shaped stop is selected to prevent the image of sunlit Earth from entering the secondary optical path.

Narrow-band filters are also used to suppress scattered light. Their full-width half-maximum passbands are typically about 1 to 2 nm. The out-of-band rejection is 10^-6. The temperatures of the filters are actively controlled by heaters and are maintained at 5 ± 1° C to obviate the variations of peak transmission wavelength with temperature.

Coatings and knife-edged baffles are carefully used in the instrument to aid in the rejection of stray light. The baffle edges in the collimator and to the position of the field stop are coated with Martin Black to minimize the scattering. Martin Black absorbs 99.5% of the incident light which can be compared to the 85 anodization. The interior sidewalls of the collimator and the internal walls of the instrument are coated with Dow 9/40. The aperture door is an integral part of the collimator and partially shields the front baffling from illumination by the sunlit Earth.