2.The laser tracker has excellent a

2.The laser tracker has excellent accuracy and sub-micron resolution for the line of sight distance, measured with its internal DMI. The angular measurements are accurate to 0.5-1 arcsecond after a standard calibration using the commercial software. For the GMT measurement, this leads to errors in lateral displacement of 50-100 µm. Ideally, the tracker would be mounted at the center of curvature of the optic, which minimizes the sensitivity to angles. In practice, this point is not accessible for some optics, including convex optics and the GMT segment, whose center of curvature is 38 m above the segment and outside the Mirror Lab. The geometry for this system is shown in Figure 5. The angle , which is the deviation of the line of sight from the surface normal, is calculated approximately for a spherical mirror, assuming small angles: