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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, 15μm pixel, fully-depleted, p-channel devices that have been thinned to ∼250μm, 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/pixel, full-well capacities ∼40,000 to 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 450nm and 980nm (≳90% between 600nm and 900nm). We optimized the readout sequence timing to achieve sub-electron noise (∼ 0.5 e− rms/pixel) in a region of 2k×4k pixels and photon-counting noise (∼0.22 e− rms/pixel) in a region of 220×4k pixels, each with a readout time of ≲17 min. We observed two Lyman-α emitting quasars (HB89 1159+123 and QSOJ1621–0042) 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 Boötes 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 improvements for SIFS observations in the low-background, readout-noise-dominated regime. We outline future scientific studies that will leverage these SIFS-Skipper CCD data, as well as new detector architectures that utilize the Skipper floating gate amplifier with faster readout times.
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