## 4 Tests  
To verify the accuracy of the theoretical calculation, vibration tests were performed to further test the mechanical stability of the DASH interferometer assembly, including sine sweep and random vibration tests. A sine sweep test was used to test the natural frequencies in three directions, and a random vibration test was used to test the mechanical stability of the assembly. Two acceleration sensors tied to the two arms of the interferometer were used to measure the acceleration response of the optical components. Figure 13 illustrates the positions of the acceleration sensors. As a load, the two sensors had a certain arm's length that was relative to the flexible structure, and an additional torque was generated during random vibration. Therefore, the finite element model was modified by adding the sensor models. Taking the test values as reference values, the relative error (\(\delta\)) of the natural frequencies in three directions between the FEM and sine sweep test results was less than 4% (\(\delta=\text{ABS}(S-T)/T\)), as summarized in Table 7.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Direction**} & \multicolumn{2}{c}{**Natural Frequencies (Hz)**} & \multirow{2}{*}{**Error (\(\delta\))**} \\ \cline{2-3} \cline{5-5}  & **FEM (S)** & & **Test (T)** \\ \hline \(X\) & 634 & 649.7 & 2.4\% \\ \(Y\) & 686 & 669.8 & 2.4\% \\ \(Z\) & 942 & 979.2 & 3.8\% \\ \hline \hline \end{tabular}
\end{table}
Table 7: Comparison of natural frequencies of modal analysis and test.  
Figure 12: Structural schematic: (**a**) existing structure; (**b**) flexible structure.  
During the random vibration tests, which require a test level of 3.535 g (root mean square value), the results show that the responses in three main vibration directions (\(X\), \(Y\), and \(Z\)) were 17.71 g, 23.4 g, and 31.91 g, respectively. Compared with the random vibration test conditions, the corresponding amplifications were 5, 6.6, and 9, respectively. The maximum acceleration response (31.91 g) was consistent with the input conditions of the mathematical model (30 g). The mathematical model results are reliable. The vibration tests indicate that the sine sweep curve was consistent before and after the vibration tests, as shown in Figure 14. The test results show that there was no breakage (metal-to-glass) of the bonding surface, and the interferometer assembly remained undamaged. The maximum mechanical stress was lower than the tensile strength of material. The shear stress at the bonding surface (metal-to-glass gluing) was less than the shear strength of the epoxy adhesive (8217).  
The interference fringe frequency did not change (the number of fringes was 50) in the optical tests before and after the vibration test, which directly reveals that there was no breakage in the bonding surfaces between the optical elements, as shown in Figure 15c. Figure 15 shows the schematic of the optical test and interferometer fringe. The shear stress at the bonding surface (glass-to-glass gluing) was less than the shear strength of the epoxy adhesive (NOA61). Therefore, the proposed flexible support structure of the DASH interferometer could meet the requirement of the launch environment.  
Figure 14: Sweep sine curve before and after the test.  
Figure 13: Photograph of the interferometer assembly in near infrared (vibration testing).