New Horizons Observations of the Cosmic Optical Background
  We used existing data from the New Horizons LORRI camera to measure the
optical-band ($0.4\lesssim\lambda\lesssim0.9{\rm\mu m}$) sky brightness within
seven high galactic latitude fields. The average raw level measured while New
Horizons was 42 to 45 AU from the Sun is $33.2\pm0.5{\rm ~nW ~m^{-2}
~sr^{-1}}.$ This is $\sim10\times$ darker than the darkest sky accessible to
the {\it Hubble Space Telescope}, highlighting the utility of New Horizons for
detecting the cosmic optical background (COB). Isolating the COB contribution
to the raw total requires subtracting scattered light from bright stars and
galaxies, faint stars below the photometric detection-limit within the fields,
and diffuse Milky Way light scattered by infrared cirrus. We remove newly
identified residual zodiacal light from the IRIS $100\mu$m all sky maps to
generate two different estimates for the diffuse galactic light (DGL). Using
these yields a highly significant detection of the COB in the range ${\rm
15.9\pm 4.2\ (1.8~stat., 3.7~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 18.7\pm 3.8\
(1.8~stat., 3.3 ~sys.)~ nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of
0.608 $\mu$m. Subtraction of the integrated light of galaxies (IGL) fainter
than the photometric detection-limit from the total COB level leaves a diffuse
flux component of unknown origin in the range ${\rm 8.8\pm4.9\ (1.8 ~stat., 4.5
~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 11.9\pm4.6\ (1.8 ~stat., 4.2 ~sys.) ~nW
~m^{-2} ~sr^{-1}}$. Explaining it with undetected galaxies requires the
galaxy-count faint-end slope to steepen markedly at $V>24$ or that existing
surveys are missing half the galaxies with $V< 30.$
