Optomechanical design of an ADC and image rotator for the HJST Coudé Spectrograph at McDonald Observatory

Abstract

High-resolution spectroscopic observations require stable slit illumination and mitigation of atmospheric refraction effects that degrade throughput and spectral fidelity. We present the optical and mechanical design of a Rotational Atmospheric Dispersion Corrector (RADC) for the coudé Tull Spectrograph on the Harlan J. Smith 2.7 m Telescope. Atmospheric dispersion introduces wavelength-dependent image displacement that increases with zenith distance and reaches approximately 4 arcsec across the 350 - 1000 nm wavelength range at a zenith angle of 65°, leading to wavelength-dependent slit losses and non-uniform slit illumination. The RADC employs a pair of counter-rotating Amici prisms to provide continuous correction over the operational zenith-angle range while maintaining image quality and compatibility with the existing coudé pre-slit optics. The optical and mechanical design of the deployable ADC subsystem, its integration within the limited relay envelope, and its expected impact on throughput and slit illumination stability are presented. In addition, we discuss a K-mirror image rotator as a potential future upgrade for observations of extended and non-point-like targets, where maintaining a fixed slit position angle on the sky may be desirable during long integrations. Preliminary optical studies of the image-rotation concept and its integration considerations within the HJST coudé relay are briefly summarized. Together, these developments support enhanced observing capabilities for high-resolution spectroscopy on the HJST.