The Near-Infrared Gatherer of Helium Transits (NIGHT)

This paper provides a comprehensive overview of the subsystems of the NIGHT instrument.NIGHT (the Near Infrared Gatherer of Helium Transits) is a narrowband, high-resolution spectrograph, marking the first dedicated survey instrument for exoplanetary atmosphere observations.Developed through a collaboration between the Observatory of Geneva, several other Swiss institutes, and the Université de Montréal, NIGHT aims to conduct an extensive statistical survey of helium atmospheres around 100+ exoplanets over several years.The instrument will report new detections of helium in exoplanet atmospheres and perform temporal monitoring of a subset of these.NIGHT measures absorption from the metastable helium state during exoplanet transits, observable in a triplet of lines around 1083.3 nm.The instrument comprises a vacuum enclosure housing the spectrograph, a front end unit for fiber injection at the telescope's focal plane, and a calibration and control rack containing calibration light sources and control hardware.The spectrograph is optimized for efficiency around the helium triplet, achieving a throughput of approximately 71%, uniform across wavelength and polarization.The primary disperser employs a volume-phaseholographic grating in a unique double-pass configuration, enabling a spectral resolution of 75 000 while maintaining high throughput.The detector is a HAWAII-1 1024 × 1024 infrared array, cooled to 85K, with the spectrograph operating at room temperature.A shortpass filter at 85 K, positioned in front of the detector, filters out longer infrared wavelengths.Thanks to its relatively high throughput, NIGHT on a 2-m class telescope is predicted to be as sensitive as existing high-resolution spectrographs on 4-m class telescopes.The front end unit injects starlight and sky background into two separate optical fibers leading to the spectrograph.It also performs near-infrared guiding and includes a mechanism for injecting calibration light into either fiber.The assembly and optical alignment of NIGHT's spectrograph and front end unit are scheduled for July-September 2024, with the first light anticipated before early 2025.Following commissioning, NIGHT is expected to begin its baseline survey, requiring 75 nights per year.