A Natural Formalism for Microlensing

If the standard microlensing geometry is inverted so that the Einstein ring is projected onto the observer plane rather than the source plane, then the relations between the observables (θ E , E ) and the underlying physical quantities ( M ,π rel ) become immediately obvious. Here θ E and E are the angular and projected Einstein radii, M is the mass of the lens, and π rel is the lens-source relative parallax. I recast the basic formalism of microlensing in light of this more natural geometry and in terms of observables. I then find that the relations between observable and physical quantities assume an exceptionally simple form. In an appendix, I propose a set of notational conventions.