Astronomical Spectroscopy with Skipper CCDs: First Results from a Skipper CCD Focal Plane Prototype at SIFS

We present the first on-sky results from an ultra-low-readout-noise Skipper CCD focal plane prototype for the SOAR Integral Field Spectrograph (SIFS). The Skipper CCD focal plane consists of four 6k × 1k, 15 μm pixel, fully-depleted, p-channel devices that have been thinned to ~250 μm, backside processed, and treated with an anti-reflective coating. These Skipper CCDs were configured for astronomical spectroscopy, i.e., a single-sample readout noise $< 4.3$\,e⁻\,rms/pixel, the ability to achieve multi-sample readout noise 1\,e⁻\,rms/pix, full-well capacities ~40,000--65,000 e⁻, low dark current and charge transfer inefficiency (~ 2 × 10⁻⁴ e⁻/pixel/s and 3.44 × 10⁻⁷, respectively), and an absolute quantum efficiency of 80% between 450\,nm and 980\,nm (90% between 600\,nm and 900\,nm). We optimize the readout sequence timing to achieve sub-electron noise (~ 0.5\,e⁻\,rms/pix) and photon-counting (~ 0.22\,e⁻\,rms/pix) readout noise over an area of 2k × 4k and 110 × 4k pixels, respectively in a readout time of 17min. We observed two Lyman-α emitting quasars (HB89\,1159$+$123 and QSO\,J1621$–$0042) at redshift z ~ 3.5, two moderate redshift galaxy clusters (CL\,J1001$+$0220 and SPT-CL\,J2040$-$4451), an emission line galaxy at $z = 0.3239$, a candidate member star of the Bo\"{o}tes~II ultra-faint dwarf galaxy, and five CALSPEC spectrophotometric standard stars (HD074000, HD60753, HD106252, HD101452, HD200654). We present charge-quantized, photon-counting observation of the quasar HB89\,1159$+$123 and show the detector sensitivity increase for faint spectral features. We demonstrate signal-to-noise performance improvement for SIFS for observations in the low-background, readout-noise-dominated regime. We outline preliminary scientific studies that will leverage the SIFS-Skipper CCD data to derive new cosmological measurements in the context of dark matter and galaxy evolution.