Optical system of Jiao Tong University spectroscopic telescope (JUST)

Jiao Tong University Spectroscopic Telescope (JUST) is a 4.8-m composite spectroscopic telescope which integrates the high-resolution observation in a small field of view (FoV) with the spectral survey in a wide FoV. The primary mirror of JUST adopts the latest segmented mirror active optics technology, which is composed of 18 hexagonal segments with a diameter of 1.1 meters each. There are two Nasmyth focal stations which can be switched by rotating the tertiary mirror. One station with a 10 arcmin FoV diameter and its image quality (full width at half maximum, FWHM) can be below 0.35 arcsec with the error budgets, which is suitable for installing high-resolution spectroscopic and imaging instruments. The other station is equipped with the wide-field correctors, which can achieve a FoV diameter of scientific 1.2 degrees and correct atmospheric dispersion. The wide-field Nasmyth focal station operates at a wavelength band ranging from 0.35 to 1.3μm and for the zenith distance ranging from 0 to 60 degrees, the image quality (FWHM) falls within a 0.75 arcsec diameter with error budgets. It is used for broad observations and mainly used for wide-field spectroscopic survey of stars, galaxies, etc. This article will describe the optical system design, the baffle design and stray light analysis, error budget, and thermal effect of the telescope in detail.