Medium-precision null-screen testing of off-axis parabolic mirrors for segmented primary telescope optics: the large millimeter telescope.

The feasibility of using null screens for testing the segments of a parabolic segmented telescope mirror for the Large Millimeter Telescope (LMT) is analyzed. An algorithm for designing the null screen for testing the off-axis segments of conic surfaces is described. Actual screen designs for the different classes of segments of the LMT are presented. The sensitivity of the test and the required accuracies for the fabrication and positioning of the screen are analyzed. A measuring accuracy of approximately 12 microm in surface sagitta is within the reach of the technique.