# Design of a Dynamic Secondary Mirror Truss Adjustment Mechanism for Large Aperture Telescopes
## 5 Experimental Validations
### Motion Accuracy Tests  
The SMAM measures the attitude accuracy after each movement step. Like the measurement of z-directional movement resolution, a micrometer is placed at the center of the secondary mirror barrel to measure the z-directional positional accuracy of the adjustment mechanism. Two micrometers are placed on each side of the secondary mirror barrel to calculate the deflection angle of the secondary mirror barrel. The height difference between the two sides of the secondary mirror barrel gives the positional accuracy of the adjustment mechanism, as shown in Figure 12. The test scheme for the position accuracy of the z-direction is as follows: Starting from -1 mm, drive the SMAM in steps of 0.1 mm within the range of -1 mm \(\sim\) +1 mm, then measure the position accuracy after achieving movement of each step. The specific measurement is taken from \(-\)1 mm to 0 mm, then +1 mm, and repeat the test five times. The specific measurement for attitude accuracy includes 0 to 400\", then 0 to +400\"and finally to 0, and repeat the test five times. Figure 11: Results of motion resolution tests for displacement along the z-direction. Figure 10: z-directional resolution test. Figure 13 shows the results of the measurements in the z-direction. The number of drive steps is the horizontal coordinate, and the displacement measurement after each movement step is according to the vertical coordinates. The actual position of each measurement is shown as a green dot and the theoretical position with a red dot. The red dot of the theoretical position is placed above the green dot of the measurement result. The greener dots in the graph give the absolute error between the actual measured and theoretical positions. If the theoretical position coincides with the actual measurement position, the red dot at that position covers the green dot and is shown as a red dot. The absolute positioning error of the measurements after each step of the movement compared to the theoretical position is analyzed, as shown in Figure 14 below. Figure 12: Deflection accuracy test with micrometers. Figure 13: Measurement results of the z-directional position accuracy of the SMAM. From Figure 14, the absolute positioning accuracy of the SMAM is better than \(\pm\)0.016 mm in the z-direction. It starts from \(-\)1 mm and moves in steps of 0.1 mm within \(-\)1 mm \(\sim\)+1 mm. The absolute value of the repetitive positioning error at each position point can be obtained by processing the measurement results of the five repetitive movements, as shown in Figure 15.  
From Figure 15, it is clear that the repeatable positioning accuracy of the SMAM is better than \(\pm\)0.01 mm for translational movements along the z-direction, within the range of \(-\)1 mm to +1 mm. The accuracy of the deflection attitude in the x-direction is measured according to the test protocol. Figure 16 shows the obtained results. Figure 16: Attitude accuracy measurements for deflection around the x-direction. Figure 14: Absolute positioning error in the z-direction at each position point. Figure 15: Repeat the positioning error in the z-direction at each position point. Figure 17 shows the results of the absolute positioning error test of the deflection attitude in the x-direction of the SMAM. According to Figure 17, the absolute positioning accuracy of the attitude angle is better than 6.4\({}^{\prime\prime}\) when the deflection in the x-direction is in the range of \(-400^{\prime\prime}\) to \(+400^{\prime\prime}\). The measurement results of the five repeated movements are used to obtain the absolute error values of the repeated positioning of the deflection attitude angle in the x-direction, as shown in Figure 18.  
The SMAM demonstrates an attitude angle repeatability accuracy better than \(\pm 1.5^{\prime\prime}\) in the range of \(-400^{\prime\prime}\) to \(+400^{\prime\prime}\)along the x-direction. The positional accuracy in other directions is also measured by the same method.