  This work presents an initial on-sky performance measurement of the
Single-Photon Imager for Nanosecond Astrophysics (SPINA) system, part of our
Ultra-Fast Astronomy (UFA) program. We developed the SPINA system based on the
position-sensitive silicon photomultiplier (PS-SiPM) detector to record both
photoelectron (P.E.) temporal and spatial information. The initial on-sky
testing of the SPINA system was successfully performed on UT 2022 Jul 10, on
the 0.7-meter aperture Nazarbayev University Transient Telescope at the
Assy-Turgen Astrophysical Observatory (NUTTelA-TAO). We measured stars with a
wide range of brightness and a dark region of the sky without stars $< 18$ mag.
We measured the SPINA system's spatial resolution to be $<232\mu m$ (full-width
half-maximum, FWHM), limited by the unstable atmosphere. We measured the total
background noise (detector dark counts and sky background) of 1914 counts per
second (cps) within this resolution element. We also performed a crosstalk
mapping of the detector, obtaining the crosstalk probability of $\sim0.18$ near
the detector's center while reaching $\sim 50\%$ at the edges. We derived a
$5\sigma$ sensitivity of $17.45$ Gaia-BP magnitude in a 1s exposure with no
atmospheric extinction by comparing the received flux with Gaia-BP band data.
For a $10ms$ window and a false alarm rate of once per 100 nights, we derived a
transient sensitivity of 14.06 mag. For a $1\mu s$ or faster time scale, we are
limited by crosstalk to a 15 P.E. detection threshold. In addition, we
demonstrated that the SPINA system is capable of capturing changes in the
stellar profile FWHM of $\pm1.8\%$ and $\pm5\%$ change in the stellar profile
FWHM in $20ms$ and $2ms$ exposures, respectively, as well as capturing stellar
light curves on the $ms$ and $\mu s$ scales.
