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 10µm,and the optimal optical performance is 2 arcsec(80%of the encircled energy diameter less than 20µm),which is suitable for the photometric survey.