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The new degrees of freedom offered by the new family of freeform surfaces helps compensating for off axis aberrations in optical designs. We define a freeform surface as a non-rotationally symmetric surface which cannot be described as an off-axis part of a conicoid. The use of freeform surfaces allows an increase in performance such as on the field of view, the F-number and compactness of unobscured all reflective telescopes. Reflective designs have the advantage of being light, achromatic, and potentially low-cost due to cheaper materials involved. It could result in industrial production of compact panchromatic systems for various applications ranging from automotive sensors to imaging nanosatellites. In this paper we address the challenges posed by the use of these surfaces in optical design optimization, specifically the minimization of the surface counting, in order to simplify the manufacturing, assembly and integration phases. We focus on three mirror designs and work on the compactness with various flavors in the 3D space. We will discuss our results on a fast, unobscured, non reimaging and well corrected three mirror anastigmat (TMA) design for a nanosatellite thermal imaging payload using uncooled bolometers. Optimization method of a TMA under volume and straylight constraints is described. A tolerance analysis is performed and minimal alignment is required even for large tolerances.
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