Upgraded optical design of a multi-band long-slit far-UV spectroscopic payload for high-resolution detection of diffuse galactic emission

Abstract

Ardua is a mission concept proposed for NASA’s ASTRA Initiative and submitted to COPAG and PhysPAG, combining Far-UV (FUV) and X-ray instruments to investigate the baryon cycle connecting stars to the cosmic web through observations of the circumgalactic medium (CGM). This proceeding focuses on the preliminary optical design of the FUV long-slit spectroscopic payload for mapping faint, diffuse CGM emission. The baseline design targets the O VI doublet within 103–104 nm, with plans to extend coverage to additional wavelength bands. It features a slit field of view of approximately 33′ × 1.6′′ and targets a spatial resolution of < 4′′ and a spectral resolving power of R ∼ 50,000, corresponding to a velocity resolution of approximately 6 km s−1. These requirements support observations of CGM structure and gas kinematics, with a surface-brightness sensitivity goal of 10−20 erg s−1 cm−2 arcsec−2. The optical system combines a 360-mm-aperture f/7 Ritchey–Chrétien telescope, a biconic collimator, a reflection grating with a central hole, and an off-axis three-mirror imager. The imager uses a confocal configuration satisfying the linear astigmatism-free condition as an analytical starting point, followed by freeform surface optimization to control aberrations across the coupled spatial and spectral fields. Ray-tracing results show a representative system RMS spot radius of approximately 2.8–4.5 μm, while the modeled dispersion yields an estimated resolving power of R ≈ 48,300–51,750 for two-pixel spectral sampling. This study explores the optical design parameter space to identify a configuration that meets these demanding science requirements. Future work will address further design refinement for practical implementation.