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)
A brief explanation of how the Optical System works is provided
below:
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 (Fig. 1). 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 (Fig. 2) 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.
