Diffractive telescope has the advantages of lightweight and loose tolerance, so it can become the next generation spacetelescope. In order to realize no central obscuration, compact structure and large field of view (FOV), the off-axis threemirror is used as eyepiece in diffractive telescope system. According to the Schupmann’s achromatic theory, an opticaldesign example of diffractive telescope with 0.8m aperture of objective lens, 0.46~0.54μm spectral bandwidth and 0.08°FOV is presented. By using the function of coordinate break in optical design software ZEMAX, an unobscureddiffractive telescope system based on off-axis three mirror eyepiece is realized. After optimized original structureparameters, the diffractive telescope system has a good performance with 39.1m short total length. Here, at the spatialfrequency of 50lp/mm, the values of modulation transfer function (MTF) within full FOV are greater than 0.574.Furthermore, the values of RMS radius are less than airy radius diameter. The optimized results clearly show that thesystem is achromatic and near diffraction limited imaging quality.
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