Development and ground-based demonstration of a CdZnTe immersion grating for space-based mid-infrared high-resolution spectroscopy

Abstract

High-resolution spectroscopy of molecular lines in the mid-infrared fingerprint region allows innovative science cases, such as surveying snowline locations in protoplanetary disks, but poor atmospheric transmission in this range hinders sensitive, continuous ground-based observations. We are proposing the High-Resolution Spectrometer (HRS) for GREX-PLUS, a 1.0-m cryogenic space telescope mission candidate of ISAS/JAXA. HRS targets 10–18 μm at a resolving power of R ∼ 30,000, requiring a significant size reduction of the optical system for spaceborne implementation. A CdZnTe (CZT) immersion grating (IG), which exploits the high refractive index of CZT (n ∼ 2.6), is a promising solution. Since CZT-IGs have not been demonstrated in astronomical spectroscopy, we are conducting verification studies to establish CZT-IGs as practical dispersive elements and developing a ground-based spectrometer to evaluate the on-sky performance of a CZT-IG. In this paper, we summarize the status of CZT-IG development and present the design of the demonstrator spectrometer. The instrument covers the N-band (7.5–13.5 μm) at R ∼ 30,000 and will be installed at the Nasmyth focus of the 1.5-m Kanata telescope. The optical design employs a CZT-IG as the main dispersive element and two cross dispersers for order scanning. Spot-diagram and echellogram analyses indicate that the aberrations are sufficiently small and that the target band can be scanned without wavelength gaps. We also outline the current mechanical and thermal design concept for cryogenic operation. This demonstrator is an essential step toward validating CZT-IG technology and identifying issues before application to GREX-PLUS/HRS.