SPIE Journal Paper | 28 July 2021
KEYWORDS: Spectrographs, Exoplanets, Planets, Spectroscopy, Earth's atmosphere, Spectral resolution, Mirrors, Sensors, Cameras, Diffraction gratings
NEREA (Near Earths and high-Res Exoplanet Atmospheres) is a stable, compact, high-resolution spectrometer concept for the Gran Telescopio Canarias (GTC). The spectrograph is designed to be a common user instrument; however, its specifications are governed by two main science goals. (1) The discovery and characterization of planets around late-M type stars. Using the GTC’s large 10 m collecting area, NEREA would be capable of reaching out for planets around cooler, smaller, and dimmer stars than current facilities, contributing to completing the census of small planets in our stellar neighborhood (<30 pc); (2) The characterization of planetary atmospheres from Hot-Jupiters to Super-Earths. NEREA would be capable of detecting species including H2O, CH4, CO2, and ionized metals in planetary atmospheres; as well as detecting and resolving individual Na I, Hα, and He I line profiles, improving our understanding of the physical and chemical evolution of planetary atmospheres. Owing to the GTC’s collecting area, the current planetary sample size can be expected to increase by ∼40 times. The spectrometer preliminary design follows a single-pass echelle layout and is fiber-fed with both a rectangular 22 μm × 198 μm science fiber and a 29 μm calibration fiber. The system is designed to achieve a resolving power of 100,000 in the red-NIR regime (0.7 to 1.7 μm). The device footprint is ∼0.25 m × 1 m × 0.75 m, allowing the system to be temperature and pressure stabilized (<50 cm / s) in a small, compact container. The concept applies a 320 mm × 90 mm, R4 blazed, diffraction grating as the disperser and a 60.6-deg P-SF67 prism as the cross disperser. A 325 mm focal length, off-axis, parabolic mirror is employed as the collimator and the camera consists of a custom designed 325 mm refocusing lens. The instrument is designed to efficiently couple to the GTC AO system, for which we assume moderate performance of 0.3″ in our wavelength range. The spectrometer would also be able to perform efficiently without adaptive optics at a resolving power of 40,000 or, with losses, at a resolving power of 70,000 to 100,000.
