High-performance thermal imagers like Mars Climate Sounder (MCS) on the Mars Reconnaissance Orbiter (MRO) and the Diviner Lunar Radiometer Experiment on the Lunar Reconnaissance Orbiter (LRO) currently use a three-mirror anastigmat (TMA) optical design to image remote targets. A TMA telescope is built with three curved mirrors, enabling it to minimize all three main optical aberrations: spherical aberration, coma, and astigmatism. This is primarily used to enable wide fields of view, much larger than possible with telescopes with just one or two curved surfaces.
The Korsch annular field three-mirror anastigmat (AFTMA) telescope is an expansion of the standard TMA. This TMA offers a wide, diffraction-limited field of view (FOV) suitable for space-based survey missions. The advantages of this configuration include superior stray light baffling (distinct exit pupil) and the wide range of available focal lengths from geometrically similar configurations. Within the acceptable limits of geometric blur (0.1 arcsec), Korsch was remarkably able to correct four aberrations (spherical aberration, coma, astigmatism, and field curvature) with three curved mirrors. Elimination of field curvature allows a flat focal surface and greatly reduced mechanical complexity in a telescope focal plane.
The precise location of the optical/thermal filter is typically in a diverging, converging, collimated, or focused beam. In current thermal imager designs, the spectral filter is placed at the detection plane, on top of the detector. The fast optical beam from the TMA (F/2) incident on the filter surface leads to thermal transient phenomena, and the necessary baffling between the filter and the detector limits the minimum size of the pixels that can be accommodated. This innovation enables the use of the next generation of smaller pixels by adding an intermediate focus to the AFTMA telescope to accommodate a spectral filter assembly.
In particular, the innovation lies in how the system is optimized to create a spatial and spectral transformation plane that creates optimized wavefront aberration correction at the focal plane, while simultaneously optimizing a purposely aberrated intermediate focus plane to match the filter geometry, and keeping the optical-system as compact as possible for deep-space applications. Filters are placed along the z axis such that the energy is spread out over a stretched x axis and narrow y direction (sagittal plane). Aberrations are thus exploited along the focused beam path (waist) to transfer power from field angles while not compromising ultimate spatial resolution quality at the detection plane. The novelty of this is that spatial aberrations are specifically encoded in the optical stream to exploit the specific filter shape. An optimization routine is used to arrive at a solution that is not optimal based on the usual metrics at the intermediate plane. The solution is optimal at the detection plane based on a wavefront error metric while optimal at the intermediate plane based on a spatial-spectral filter geometrical metric. This metric allows the power to be transferred and spectrally filtered while maintaining the same high-performance wavefront aberration at the detector plane.
In addition, the differential magnification at the intermediate plane (F/4) compared to the focal plane (F/2) that is built into the design allows: (1) the use of 2 times larger physical filters, (2) reduced thermal transients thanks to a 20 times smaller range of incident angles on the filter block, and (3) a simpler baffle design separated from the focal plane. This allows the power to be transferred through the filter without crosstalk between spatially independent focused elements. The focused beam is thus optimized at the detector plane with zero overlap from the spatially aberrated filter beam.
This innovative design is an ultra-compact, high-performance, spectral filter imaging system for earth and space science specifically developed for the next-generation MCS/DIVINER thermopile imager. The design has minimized transient thermal phenomena, a 2 times larger filter assembly, and high-performance imaging at the detector array compatible with ever smaller pixels.