  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 x 1k, 15 $\mu$m pixel,
fully-depleted, p-channel devices that have been thinned to ~250 $\mu$m,
backside processed, and treated with an anti-reflective coating. These Skipper
CCDs were configured for astronomical spectroscopy, i.e., single-sample readout
noise < 4.3 e- rms/pixel, the ability to achieve multi-sample readout noise
$\ll$ 1 e- rms/pixel, full-well capacities ~40,000-65,000 e-, low dark current
and charge transfer inefficiency (~2 x 10$^{-4}$ e-/pixel/s and 3.44 x
10$^{-7}$, respectively), and an absolute quantum efficiency of $\gtrsim$ 80%
between 450 nm and 980 nm ($\gtrsim$ 90% between 600 nm and 900 nm). We
optimized the readout sequence timing to achieve sub-electron noise (~0.5 e-
rms/pixel) in a region of 2k x 4k pixels and photon-counting noise (~0.22 e-
rms/pixel) in a region of 220 x 4k pixels, each with a readout time of
$\lesssim$ 17 min. We observed two quasars (HB89 1159+123 and QSO J1621-0042)
at redshift z ~ 3.5, two high-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 observations 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 scientific studies that will leverage the SIFS-Skipper CCD data and new
detector architectures that utilize the Skipper floating gate amplifier with
faster readout times.
