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A mid-to-high resolution optical payload, named STREEGO, featuring a compact envelope, reduced mass, and competitive cost has been designed to address Earth Observation applications from low Earth orbit (LEO) small satellites. STREEGO is a fully reflective telescope designed for operation at a nominal altitude of 600 km to provide a ground sampling distance (GSD) of 2.75 m, a field of view of about 1° and a modulation transfer function (MTF) greater than 10% at Nyquist frequency (91 cy/mm) with a telescope aperture of 200 mm. A large two-dimensional CMOS sensor with a pixel size of 5.5 m has been selected in order to obtain the requested GSD with a focal length of 1.2 m, and a signal-to-noise ratio equal to 75. A demonstration model is in the detailed design phase and will be developed, with completion planned by end of 2015. We present a detailed description of the instrument, which, by leveraging on aspheric surfaces and light-weight solutions, achieves remarkable performance in terms of compactness and image quality with less than 25 kg in mass. The optical design is based on a three mirror anastigmat (TMA) configuration whereby the mirrors are realized in metal with the same coefficient of thermal expansion of the structure in order to obtain an athermal design.