  Recent advances in cosmic-ray detectors have provided exceptional
sensitivities of dark matter with high angular resolution. Motivated by this,
we present a comprehensive study of cosmic-ray flux from dark matter decay in
dwarf spheroidal galaxies (dSphs), with a focus on detectors possessing
arcsecond-level field of view and/or angular resolution. We propose to use
differential $D$-factors, which are estimated for various dSphs since such
detectors are sensitive to their dark matter distributions. Our findings reveal
that the resulting signal flux can experience a more than $O$(1-10) enhancement
with different theoretical uncertainty compared to traditional estimations.
Based on this analysis, we find that the Infrared Camera and Spectrograph
(IRCS) installed on the 8.2m Subaru telescope can be a good dark matter
detector for the mass in the eV range, particularly axion-like particles
(ALPs). Observing the Draco or Ursa Major II galaxies with the IRCS for just a
few nights will be sufficient to surpass the stellar cooling bounds for ALP
dark matter with a mass in the range of $1\,{\rm eV} \lesssim m_a \lesssim
2\,\rm eV$.
