An unobscured optical system provides a good solution for a high-performance diffractive telescope. However, the unobscured diffractive telescope, such as an off-axis three-mirror diffractive telescope (OTDT), suffers from the complex calculation of initial parameters, strict tolerance, and fabrication of aspheric mirror diffractive optical elements (DOE). In this paper, a simplified unobscured design method, which is effective in achieving compact and loose tolerance, is proposed. Combining the deflection of an optical path and Schupmann achromatic theory, a novel unobscured diffractive telescope is rapidly converted from a coaxial transmissive system. Not only is the loose tolerance obtained, but the fabrication of an aspheric mirror DOE is avoided. The designed system, with a focal length of 400 mm, f -number of 5, and working waveband of 612.8-652.8 nm, is analyzed. The results show that the imaging quality approached the diffraction limit within a field of view of 0.04 degrees x 0.02 degrees. Compared with theOTDT, the designed system has great advantages in design step, DOEfabricating, and system alignment. It also provides a reference for unobscured diffractive telescope development. (C) 2020 Optical Society of America
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