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A galaxy cluster acts as a cosmic telescope over background galaxies but alsoas a cosmic microscope of the lens imperfections. The diverging magnificationof lensing caustics enhances the microlensing effect of substructure presentwithin the lensing mass. Fine-scale structure can be accessed as a movingbackground source brightens and disappears when crossing these caustics. Therecent recognition of a distant lensed star near the Einstein radius of thegalaxy cluster MACSJ1149.5+2223 (Kelly et al. 2017) allows the rare opportunityto reach subsolar mass microlensing through a super-critical column of clustermatter. Here we compare these observations with high-resolution ray-tracingsimulations that include stellar microlensing set by the observed intraclusterstarlight and also primordial black holes that may be responsible for therecently observed LIGO events. We explore different scenarios with microlensesfrom the intracluster medium and black holes, including primordial ones, andexamine strategies to exploit these unique alignments. We find that the bestconstraints on the fraction of compact dark matter in the small-mass regime canbe obtained in regions of the cluster where the intracluster medium plays anegligible role. This new lensing phenomenon should be widespread and can bedetected within modest-redshift lensed galaxies so that the luminosity distanceis not prohibitive for detecting individual magnified stars. Continuous {\itHubble Space Telescope} monitoring of several such optimal arcs will berewarded by an unprecedented mass spectrum of compact objects that cancontribute to uncovering the nature of dark matter.

Dark matter under the microscope: Constraining compact dark matter with caustic crossing events