We present an analysis of the impact of space charge effects on the performance of a proposed scanning tunneling microscope (STM) microlens with magnetic focusing.1We compare analytical and numerical estimates of energy broadening, and consider scaling of the energy broadening with magnetic field. We find relative energy spreads of 10−4to 10−3at 0.1 &mgr;A current, and 10−3to 10−2at 1 &mgr;A, implying that space charge‐induced energy broadening will not become significant compared to other sources until the current exceeds several &mgr;A. It is observed that space charge effects decrease with increasing magnetic field. For typical operating values, the space charge contribution to the beam diameter should range from 1–100 A˚.