The fourth flight of CHESS: spectral resolution enhancements for high-resolution FUV spectroscopy

In this proceeding, we describe the scientific motivation and technical development of the Colorado Highresolution Echelle Stellar Spectrograph (CHESS), focusing on the hardware advancements and testing of components for the fourth and final launch of the payload (CHESS-4). CHESS is a far ultraviolet rocket-borne instrument designed to study the atomic-to-molecular transitions within translucent cloud regions in the interstellar medium. CHESS is an objective echelle spectrograph, which uses a mechanically-ruled echelle and a powered (f/12.4) cross-dispersing grating; it is designed to achieve a resolving power R > 100,000 over the band pass λλ 1000–1600 A. CHESS-4 utilizes a 40 mm-diameter cross-strip anode readout microchannel plate detector, fabricated by Sensor Sciences LLC, to achieve high spatial resolution with high global count rate capabilities (∼ MHz). An error in the fabrication of the cross disperser limited the achievable resolution on previous launches of the payload to R ∼ 4000. To remedy this for CHESS-4, we physically stress the echelle grating, introducing a shallow toroidal curvature to the surface of the optic. Preliminary laboratory measurements of the resulting spectrum show a factor of 4–5 improvement to the resolving power. Results from final efficiency and reflectivity measurements for the optical components of CHESS-4 are presented, along with the pre-flight laboratory spectra and calibration results. CHESS-4 launched on 17 April 2018 aboard NASA/University of Colorado Boulder sounding rocket mission 36.333 UG. We present flight results for the observation of the γ Ara sightline.