1.PRIOR TRACKER MEASUREMENTS AND LI

1.PRIOR TRACKER MEASUREMENTS AND LIMITATIONSThe use of the laser tracker for surface measurements builds on the experience from measuring the figure of the1.7m off-axis primary mirror of the New Solar Telescope (NST) at Big Bear Solar Observatory.4 The NST mirror is very nearly a 1/5 scale model of the off-axis GMT segment, and served as a prototype for GMT fabrication and testing. The grinding and initial polishing of this surface was guided with the laser tracker exclusively. In the NST measurements, there was no attempt to improve the tracker’s angular accuracy or to compensate for rigid-body motion or index variations. The tracker was mounted on the same structure that held the mirror, about 2 m above the mirror. This arrangement minimized relative motion between the mirror and the tracker, but placed the tracker far from the center of curvature (R = 7.7 m), so the measurement was sensitive to angular errors. We found significant systematic errors in the tracker measurement that were reduced by averaging multiple measurements with the mirror rotated relative to the tracker. Figure 2 shows tracker data taken near the beginning of loose-abrasive grinding, with a measured error of 14 µm rms surface, and data measured near the end. The astigmatism seen in the early measurement was repeatable and was gradually reduced through figuring.