  Primordial black holes (PBHs), hypothesized to be the result of density
fluctuations during the early universe, are candidates for dark matter. When
microlensing background stars, they cause a transient apparent enhancement of
the flux. Measuring these signals with optical telescopes is a powerful method
to constrain the PBH abundance in the range of $10^{-10}\,M_{\odot}$ to
$10^{1}\,M_{\odot}$. Especially for galactic stars, the finiteness of the
sources needs to be taken into account. For low PBH masses (in this work
$\lesssim 10^{-8}\,M_{\odot}$) the average duration of the detectable event
decreases with the mass $\langle t_e\rangle \propto M_{\mathrm{PBH}}$. For
$M_{\mathrm{PBH}}\approx 10^{-11}\,M_{\odot}$ we find $\langle t_e\rangle
\lesssim\,1 \mathrm{s}$. For this reason, fast sampling detectors may be
required as they could enable the detection of low mass PBHs. Current limits
are set with sampling speeds of 2 minutes to 24 hours in the optical regime.
Ground-based Imaging Atmospheric Cherenkov telescopes (IACTs) are optimized to
detect the $\sim$ns long optical Cherenkov signals induced by atmospheric air
showers. As shown recently, the very-large mirror area of these instruments
provides very high signal to noise ratio for fast optical transients ($\ll
1\,$s) such as asteroid occultations. We investigate whether optical
observations by IACTs can contribute to extending microlensing limits to the
unconstrained mass range $M_{\mathrm{PBH}}<10^{-10}M_\odot$. We discuss the
limiting factors to perform these searches for each telescope type. We
calculate the rate of expected detectable microlensing events in the relevant
mass range for the current and next-generation IACTs considering realistic
source parameters.
