The Circumgalactic Hα Spectrograph (CHαS). I. Design, Engineering, and Early Commissioning

The Circumgalactic H alpha Spectrograph (CH alpha S) is a ground-based optical integral field spectrograph designed to detect ultrafaint extended emission from diffuse ionized gas in the nearby universe. CH alpha S is particularly well suited for making direct detections of tenuous H alpha 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 alpha S as a facility instrument at MDM Observatory. CH alpha 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 alpha S is a pupil-imaging spectrograph employing a microlens array to divide the field of view into >60,000 spectra. CH alpha 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 alpha 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 volume phase holographic grating ensures high throughput across configurations. CH alpha 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.