The Circumgalactic Hα Spectrograph (CHαS). I. Design, Engineering, and Early Commissioning
Nicole Melso, David Schiminovich, Brian Smiley, Hwei Ru Ong, Bárbara Cruvinel Santiago, Meghna Sitaram, Ignacio Cevallos Aleman, S. Graber, Marisa Murillo, Marni Rosenthal, Ioana Stelea
Abstract
Abstract The Circumgalactic H α Spectrograph (CH α S) is a ground-based optical integral field spectrograph designed to detect ultrafaint extended emission from diffuse ionized gas in the nearby universe. CH α S is particularly well suited for making direct detections of tenuous H α emission from the circumgalactic medium (CGM) surrounding low-redshift galaxies. It efficiently maps large regions of the CGM in a single exposure, targeting nearby galaxies ( d < 35 Mpc) where the CGM is expected to fill the field of view. We are commissioning CH α S as a facility instrument at MDM Observatory. CH α S is deployed in the focal plane of the Hiltner 2.4 m telescope, utilizing nearly all of the telescope’s unvignetted focal plane (10′–15′) to conduct wide-field spectroscopic imaging. The catadioptric design provides excellent wide-field imaging performance. CH α S is a pupil-imaging spectrograph employing a microlens array to divide the field of view into >60,000 spectra. CH α S achieves an angular resolution of [1.3–2.6] arcseconds and a resolving power of R = [10,000–20,000]. Accordingly, the spectrograph can resolve structure on the scale of 1–5 kpc (at 10 Mpc) and measure velocities down to 15–30 km s −1 . CH α S intentionally operates over a narrow (30 Å) bandpass; however, it is configured to adjust the central wavelength and target a broad range of optical emission lines individually. A high–diffraction efficiency volume phase holographic grating ensures high throughput across configurations. CH α S maintains a high grasp and moderate spectral resolution, providing an ideal combination for mapping discrete, ultralow–surface brightness emission on the order of a few milli-Rayleigh.