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The paper describes the challenges faced during the optical design of a multispectral imager in the Mid Wave InfraRed (MWIR) based on super-resolution and compressive sensing, performed in the framework of the SISSI project funded by the Italian Space Agency. The project aimed to improve ground sampling distance and to mitigate possible saturation/blooming effects. The operating principle is the one of the single pixel camera. The use of CS architecture allows the scene reconstruction without loss of significant information by using a reduced number of acquisitions. A Digital Micromirror Device (DMD) was used to modulate the observed image by using a binary ON-OFF pattern. A sequence of such spatially-coded acquisitions is integrated by a condenser lens and focused on the 2D detector array, each element of which acting, in parallel, as the single element detector of a single pixel camera. Multispectral acquisition is obtained by deposition of different band pass filters on the 2D detector. The optical design has posed strict constraints on the optics involved. The collection optics, focusing on the DMD plane, must match with the condensing optics. An additional issue is due to the DMD working in reflection mode. A further requirement is given by the Airy disk size. Such constraints lead to an upper limit for the optics f-number. Chromatic aberration introduces further difficulties in glasses selection with high transmissivity in the MWIR. The final optical design of the system involves both reflective and dielectric optical elements, making use of aspheric and free-form surfaces.
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