Optical design of an ultraviolet hybrid refractive-diffractive space telescope for photometry

Space-based telescopes are important tools in astronomy and Earth observations. They enable observation in spectral ranges outside of the atmospheric window, e.g. below 300 nm. One of the ways to decrease the mass of space telescopes is to use diffractive optical elements. They have unique capabilities when it comes to aberration corrections. By combining refractive and diffractive components it is possible to obtain a well-corrected system with fewer optical elements compared to purely refractive systems. In this paper we present an optical design of a hybrid refractive-diffractive telescope working in the 200 nm – 300 nm spectral range with improved performance and decreased mass compared to refractive system. The telescope has a large field of view 10°×10°, enabling observations of many objects simultaneously, focal length of 150 mm and f-number equal to F/1.67. We compare the performance of two systems optimized using different merit functions. In the first case the goal was to minimize spot size, in the other a widened point spread function was obtained in order to avoid undersampling. The results of tolerance analysis prove that the satisfactory imaging quality may be obtained with moderate tolerances. Moreover, the influence of the antireflective coating on the efficiency of the diffractive lens is discussed. Performed simulations show that the antireflective coating deposited on the diffractive structure gives an increase of the efficiency at the expected level.