Slit-referenced auto-guiding system for the spectrograph of the SiTian Pilot-Node Telescope

Abstract

Long-exposure spectroscopic observations require the target to remain accurately aligned with the center of the spectrograph slit throughout the exposure. Telescope tracking errors, structural flexure, and environmental disturbances can cause slow image drift on the focal plane. This drift reduces slit throughput and degrades spectral data quality. To address this problem, this paper presents an automatic guiding system for the low-resolution spectrograph of the SiTian pilot-node telescope. The system uses the slit-view guiding optical path to obtain real-time images of the relative position between the target and the slit. It combines image processing and closed-loop telescope control for automatic slit acquisition and stable target retention. In the image-processing module, the system detects slit edges in a region of interest, extracts the stellar centroid under truncated or split image conditions, determines the in-slit state, and converts the measured image-plane deviation into PulseGuide commands for the telescope control system. Offline tests show sub-pixel centroiding repeatability. A 30.5 min closed-loop guiding test shows continuous target detection, an in-slit ratio of 95.0%, and a cross-slit RMS deviation of 3.18 pixels, corresponding to 0.88 arcsec.