The SiTian project is mainly aimed at a photometric survey of the 10,000 square degrees sky within 30 min in three bands. The telescopes for the SiTian array must have a super-wide field-of-view (FOV). For the optical system discussed in this paper, we have investigated four types of wide-field telescope optics: Cassegrain with correctors, three-mirror anastigmatic, prime focus, and modified Schmidt types. Weighing up the pros and cons, the catadioptric system, transformed from a modified Schmidt telescope, was adapted for the prototype of the SiTian array.

The optics of the SiTian array is a 1 m/1.27 m modified Schmidt optical system, with an aperture of 1 m, an FOV of 7°(25 square degrees), and a focal ratio of 2.06, and is capable of imaging from 400 to 1000 nm. The image scale is 1 arcsec per 10 µm, and the optimal optical performance is 2 arcsec (80% of the encircled energy diameter less than 20 µm), which is suitable for the photometric survey.