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.

The Sun Yat-Sen University (SYSU) 1.2-m Multi-Terminal Telescope is a versatile astronomical facility with three-channel photometric and low-/high-resolution spectrometric functions. The SYSU telescope comprises a Ritchey-Chretien Cassegrain bi-reflection system with a working waveband ranging from 300 to 1000 nm. Within the field-of-view (FOV) of 15′, the image quality represented in 80% encircled light energy is concentrated within 0.3 arcsec.

A Cassegrain instrument unit is mounted on a telescope derotator that enables working modes to be switched among four instruments by a central switching mirror. The equipped instruments include a three-channel photometric terminal, a 1.25-inch visual observation terminal, a wavelength calibration unit, a long-slit spectrograph (LSS), and a fiber adaptor for high-resolution observation. Therein, the photometric terminal can perform simultaneous imaging in the b(320-450 nm), g(480-700 nm), and r(730-1000 nm) bands. By adding a set of corrective lenses, the visual observation terminal can also realize a large FOV of 0.76°. The LSS can provide high-throughput observation, with the resolution ranging from 1000 to 3000 by slit width. A high-resolution spectrograph (HRS) at the laboratory is fed by a 25-m fiber from the Cassegrain fiber adaptor. The resolution R of the HRS exceeds 30,000 to cover a wide band from 400 to 900 nm. The SYSU 1.2-m Multi-Terminal Telescope combines excellent photometric and spectrometric observational performances. The novel design of the SYSU telescope allows for a singular telescope to make contributions in both astronomical education and scientific research.