2008ApJ...684...46D

In a variety of astronomical situations, there is a relatively high probability that a single isolated lens will produce a detectable event. The high probability is caused by some combination of a large Einstein angle, fast angular motion, and a dense background field. We refer to high-probability lenses as ``mesolenses.'' Planetary and stellar masses located within 1-2 kpc are examples of mesolenses. We show that their presence can be detected against a wide variety of background fields, using signatures of both time variability and spatial effects. Time signatures can be identical to those of microlensing, but can also include baseline jitter, extreme apparent chromaticity, photometric events well fit by lens models in which several independent sources are simultaneously lensed, and sequences of events. Spatial signatures include sequential astrometric lensing of surface brightness fluctuations, as well as patterns of time variability that sweep across the background field as the lens moves. Wide-field monitoring programs, such as Pan-STARRS and LSST, are well suited to the study of nearby masses. In addition, targeted high-resolution observations of the region behind a known mass traveling across a dense background field can use lensing effects to measure the lens mass and study its multiplicity.