Optical System
Description
The Optical System is the core of the MODIS instrument –
it gathers, directs, and splits Earth’s radiation into
the 36 spectral bands on which MODIS focuses. It is composed
of fourteen components:
•The Afocal Telescope
• The Scan Mirror
•The Fold Mirror
• The Afocal Gregorian Telescope
• A field stop
• The Dichroic Housing
• The Secondary Mirror
• Three Dichroic Beamsplitters
• Four Focal Plane Assemblies (FPAs)
Technical Description
The double-sided Scan Mirror reflects Earth’s radiation,
or “scene energy,” over a scan range of ±55
degrees into the Afocal Telescope. Then, the Telescope’s
Fold Mirror reflects light into a plane perpendicular to the
scan plane, which cancels polarization between the Scan Mirror
and the Fold Mirror. The Afocal Gregorian Telescope, with
a magnification of four, then combines two off-axis confocal
parabolas to give a large field of view. Performance-damaging
non-scene illumination is reduced by the field stop’s
limiting of the field of view, and is located between the
primary and secondary mirrors. The scene energy then passes
above the Dichroic Housing before reflecting on the
secondary mirror and then falling on dichroic element #1.
A series of three dichroic beamsplitters separate the scene
energy into four spectral regions:
• Scene energy reflecting from dichroic#1 passes through dichroic #2 to be captured by the near-infrared FPA.
• Light reflected from the surface of dichroic #2 is directed to the visible FPA.
• Scene energy which has passed dichroic #1 is further split by dichroic #3 and sends the resulting long-wave portion through the Radiative Cooler, which houses the FPA serving wavelengths greater than 6µm (LWIR).
• Scene energy reflected from the surface of dichroic #3 proceeds to the short/mid-wave infrared objective assembly and into the Radiative Cooler to the SWIR/MWIR FPAs.
The visible and near-infrared objective assemblies each have three optical elements held in cylindrical aluminum housings. The visible lens elements are made from sapphire, SF11 and fused silica. The near-infrared lens elements utilize SK4G13 and fused silica. The short/mid-wave objective assembly uses five lens elements made from zinc selenide. Also included in the SWIR/MWIR objective is an intermediate filter assembly designed to minimize internal reflections. The long-wave assembly contains a collection of five lens elements made from cadmium-telluride, zinc selenide, and germanium. All four FPAs include filter/mask assemblies, which are held in beryllium bezels in close proximity to the detector surface.
