Nearly all ionospheric heaters operate at vertical incidence. Oblique waves cannot satisfy frequency-matching conditions necessary to excite the parametric decay instability, and they are weakened by geometric spreading below the ionosphere. Those spreading losses are mitigated, however, by focussing near caustics. This paper calculates fields near the caustics of oblique waves and estimates the corresponding increases in electron temperature. It finds that a transmitter having a power-gain product of 5 MW can raise the temperature of f-Layer electrons by a hundred degrees or more. Those temperature increases are initially concentrated in narrow regions near caustics, but spread by heat conduction over tens of kilometers.