Initial On-Sky Performance Testing of the Single-Photon Imager for Nanosecond Astrophysics (SPINA) System

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 temporal and spatial information of detected photons. The initial on-sky testing of the SPINA system was conducted on UT 10 July, 2022, with the Nazarbayev University Transient Telescope at the Assy-Turgen Astrophysical Observatory (NUTTelA-TAO), studied stars with a wide range of brightness and a dark region of the sky without stars
${ < }18$
mag. This on-sky testing yielded crucial calibration metrics for the SPINA system, demonstrating the SPINA system’s ability in rapid astronomical detections. Key findings include a spatial resolution of
${ < }232 ~\mu \text {m}$
, affected by atmospheric conditions; a background noise level of 1914 counts per second (cps) within this resolution element; and crosstalk probability of
${\sim }0.18$
near the detector’s center while reaching
${\sim }0.5$
at the edges. We derived a
$5\sigma $
sensitivity of 17.45 Gaia BP magnitude in a 1-s exposure with no atmospheric extinction. Based on a false alarm rate of once per 100 nights, the SPINA system provides a transient sensitivity of 14.06 mag on a 10-ms window and a 15 photoelectron (P.E.) detection threshold for sub-
$\mu \text {s}$
time scale, limited by crosstalk. In addition, the SPINA system proved its capability to detect rapid alterations in the stellar profile: a variation of
${\pm }1.8\%$
in the stellar profile full-width half-maximum (FWHM) under 20-ms exposure and
${\pm }5\%$
change under 2-ms exposures, as well as capturing stellar light curves on the ms and
$\mu \text {s}$
scales.