Optical wafer steppers are extremely precise instruments with some very stringent demands placed upon them. They include three-shift production operation, submicron resolution and overlay and high wafer throughput. Registration, a component of overlay, is dependent on the stability of the alignment system and its relationship to the optical axis of the reduction lens. Baseline, the distance between the alignment system and the optical axis, must be stable to for proper operation of the system. Baseline stability in the DSW Wafer Stepper® system has been a issue for a very long time. A large component of this drift has been traced to a high thermal sensitivity of a flexure in the focus motion actuator. This thermal sensitivity, coupled with the Abbe offset in the alignment microscope and the proximity of the voice coil motor to the flexure, causes a rotation of the optical column. Because of the Abbe offset, this rotation causes a shift in the baseline, which has been previously attributed to other unrelated items. Remedies for this problem are quite simple once the phenomenon is understood. They include: proper setup of the instrument to minimize the heat generated at the voice coil motor; thermal isolation of the flexure from the voice coil motor with low thermal conductivity materials; redesign of the voice coil motor for higher efficiency; and active control of the motor temperature. Later designs of the focus motion assembly eliminate the baseline drift problem through the use of high efficiency motors and mechanical design symmetry.