  CMOS detectors offer many advantages over CCDs for optical and UV
astronomical applications, especially in space where high radiation tolerance
is required. However, astronomical instruments are most often designed for low
light-level observations demanding low dark current and read noise, good
linearity and high dynamic range, characteristics that have not been widely
demonstrated for CMOS imagers. We report the performance, over temperatures
from 140 - 240 K, of a radiation hardened SRI 4Kx2K back-side illuminated CMOS
image sensor with surface treatments that make it highly sensitive in blue and
UV bands. After suppressing emission from glow sites resulting from defects in
the engineering grade device examined in this work, a 0.077 me-/s dark current
floor is reached at 160 K, rising to 1 me$^-$/s at 184 K, rivaling that of the
best CCDs. We examine the trade-off between readout speed and read noise,
finding that 1.43 e$^-$ median read noise is achieved using line-wise digital
correlated double sampling at 700 kpix/s/ch corresponding to a 1.5 s readout
time. The 15 ke$^-$ well capacity in high gain mode extends to 120 ke$^-$ in
dual gain mode. Continued collection of photo-generated charge during readout
enables a further dynamic range extension beyond $10^6$ e$^-$ effective well
capacity with only 1% loss of exposure efficiency by combining short and long
exposures. A quadratic fit to correct for non-linearity reduces gain correction
residuals from 1.5% to 0.2% in low gain mode and to 0.4% in high gain mode.
Cross-talk to adjacent pixels is only 0.4% vertically, 0.6% horizontally and
0.1% diagonally. These characteristics plus the relatively large 10 $\mu$m
pixel size, quasi 4-side buttability, electronic shutter and sub-array readout
make this sensor an excellent choice for wide field astronomical imaging in
space, even at FUV wavelengths where sky background is very low.
