The Circumgalactic H$\alpha$ Spectrograph (CH$\alpha$S) is a ground-basedoptical integral field spectrograph designed to detect ultra-faint extendedemission from diffuse ionized gas in the nearby universe. CH$\alpha$S isparticularly well suited for making a direct detection of tenuous H$\alpha$emission from the circumgalactic medium (CGM) surrounding low-redshiftgalaxies. 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 thefield of view. We are commissioning CH$\alpha$S as a facility instrument at MDMObservatory. CH$\alpha$S is deployed in the focal plane of the Hiltner2.4-meter telescope, utilizing nearly all of the telescope's unvignetted focalplane (10 arcmin) to conduct wide-field spectroscopic imaging. The catadioptricdesign provides excellent wide-field imaging performance. CH$\alpha$S is apupil-imaging spectrograph employing a microlens array to divide the field ofview into $> 60,000$ spectra. CH$\alpha$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$\alpha$Sintentionally operates over a narrow (30 Angstrom) bandpass; however, it isconfigured to adjust the central wavelength and target a broad range of opticalemission lines individually. A high diffraction efficiency VPH grating ensureshigh throughput across configurations. CH$\alpha$S maintains a high grasp andmoderate spectral resolution, providing an ideal combination for mappingdiscrete, ultra-low surface brightness emission on the order of a fewmilli-Raleigh.

Galaxies in the local Universe are thought to require ongoing replenishmentof their gas reservoir in order to maintain the observed star formation rates.Cosmological simulations predict that such accretion can occur in both adynamically hot and cold mode. However, until now observational evidence of theaccretion required to match the observed star formation histories is lacking.This paper attempts to determine whether galaxies in the local Universe possessa significant reservoir of HI and what would be the accretion rates derivedfrom such reservoirs. We search the vicinity of 22 nearby galaxies for isolatedHI clouds or distinct streams in a systematic and automated manner. The HALOGASobservations represent one of the most sensitive and detailed HI surveys todate. These observations typically reach column density sensitivities of 10^19cm^-2 over a 20 km/s width. We find 14 secure HI cloud candidates without anobserved optical counterpart. These cloud candidates appear to be analogues tothe most massive clouds detected around the Milky Way and M31. However, onaverage their numbers seem significantly reduced. We constrain upper limits forHI accretion in the local Universe. The average HI mass currently observedamounts to a rate of 0.05 Msun/yr with a stringent upper limit of 0.22 Msun/yr,confirming previous estimates. This is much lower than the average starformation rate in this sample. Our best estimate, based on GBT detection limitsof several galaxies, suggests that another 0.04 Msun/yr could be accreted fromundetected clouds and streams. These results show that in nearby galaxies HI isnot being accreted at the same rate as stars are currently being formed. Ourstudy can not exclude that other forms of gas accretion are at work. However,these observations also do not reveal extended neutral gas reservoirs aroundmost nearby spiral galaxies.