The Circumgalactic H$\hat{I}\pm$ Spectrograph (CH$\hat{I}\pm$S) I. Design, Engineering, and Early Commissioning

Abstract

The Circumgalactic H$\hat{I}\pm$ Spectrograph (CH$\hat{I}\pm$S) is a ground-based optical integral field spectrograph designed to detect ultra-faint extended emission from diffuse ionized gas in the nearby universe. CH$\hat{I}\pm$S is particularly well suited for making a direct detection of tenuous H$\hat{I}\pm$ 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$\hat{I}\pm$S as a facility instrument at MDM Observatory. CH$\hat{I}\pm$S is deployed in the focal plane of the Hiltner 2.4-meter telescope, utilizing nearly all of the telescope's unvignetted focal plane (10 arcmin) to conduct wide-field spectroscopic imaging. The catadioptric design provides excellent wide-field imaging performance. CH$\hat{I}\pm$S is a pupil-imaging spectrograph employing a microlens array to divide the field of view into $>60,000$ spectra. CH$\hat{I}\pm$S achieves an angular resolution of $[1.3-2.8]$ 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. CH$\hat{I}\pm$S intentionally operates over a narrow (30 Angstrom) bandpass; however, it is configured to adjust the central wavelength and target a broad range of optical emission lines individually. A high diffraction efficiency VPH grating ensures high throughput across configurations. CH$\hat{I}\pm$S maintains a high grasp and moderate spectral resolution, providing an ideal combination for mapping discrete, ultra-low surface brightness emission on the order of a few milli-Raleigh.

25 pages, 14 figures, 7 tables, Accepted for publication in ApJ