1.Measure astigmatism to 0.5 µm rms surface•corresponds to 1 mm change in off-axis distance•correctable with 6 N rms force2.Measure coma to 0.14 µm rms surface•correctable with 20 N rms force3.Measure defocus to 0.3 µm rms.•equiv to 0.2 mm radius error•corresponds to 0.4 mm change in off-axis distance•correctable with 10 N rms forceMeasure distance from tracker to segment to 0.2 mm.