A direct means of measuring the aerial image profile of a microlithographic stepper system is described. The position of the peak gives the relative overlay error and the shape of the curve relates to the resolution. Reduction and focus shifts due to barometric pressure change are reported. Vibration between the aerial image and the wafer was measured, as well as the effects of vibration on resist imagery. We also describe applications of such techniques for fast and accurate setup of stepper overlay offset and focus.