A laser tracker is a commercial device that measures the position of a retroreflector in 3 dimensions by using a distance-measuring interferometer (DMI) and two angular encoders. It is sensitive to sub-micron displacements in the radial direction, and the encoder accuracy is on the order of 1 arcsecond. It is capable of measuring a mirror surface to sub-micron accuracy if the tracker is located at the center of curvature, so angular errors do not affect the surface measurement, and if rigid-body motion of the mirror and tracker can be controlled during the course of the measurement, typically several minutes to an hour. We have demonstrated this sort of accuracy in measurements of a 1.7 m off-axis mirror.4 In order to achieve similar accuracy with a much larger mirror, we need to improve the accuracy of the angle measurements and add stability references that compensate for rigid- body motion and large-scale variations in refractive index.