Stray light analysis and control for the Wide Field Survey Telescope (WFST)

Abstract

The wide field survey telescope (WFST) is a 2.5 meter optical telescope that is currently under construction in China. Designed with a large field of view (FOV) of 3 degrees in diameter and equipped with a 0.75 gigapixel mosaic CCD camera, the telescope will be mainly used for high sensitivity time-domain imaging surveys across the northern sky. The optical design for WFST features an advanced primary-focus assembly (PFA) housing five corrector lenses, an atmospheric dispersion corrector, filters of six bands, and the CCD camera. Stray light rejection performance is crucial for WFST to achieve an optimal sensitivity and maximize its scientific outputs. The primary-focus geometry of WFST helps to reduce the celestial background compared with a Cassegrain geometry, but the wide FOV imposes additional difficulty in stray light control and suppression. In this paper, the stray light behavior of WFST is carefully modeled by establishing a detailed opto-mechanical model of the telescope, assigning proper surface properties, and launching ray tracing simulations for a variety of scenarios. Important stray light paths including ghost effect and first-order scatterings are identified. Stray light mitigation measures including baffle and mask designs are proposed and optimized based on the stray light analysis results, which show promising suppression capability.