# Large-Aperture Real-Time Compensated Collimating Wavefront Error Detection Method
## 3 Experiment and Analysis
The corresponding deviation data could be obtained through the real-time compensation system. Additionally, there was a tilting-related error in the data before compensation, which generally resulted from the linear motion error of the guide rail. The wavefront under measurement came from the transfer function source with a 300 mm aperture. According to the manufacturer's datasheet, the values of PV and root mean square (RMS)  
Figure 4: Wavefront fitting results with (**a**) pre-compensated data and (**b**) post-compensated data. Figure 3: Experiment device diagram. for the full-aperture wavefront were 0.440 \(\lambda\) and 0.074 \(\lambda\), respectively. Three pentaprism scanning detection methods: A traditional scanning method, a double pentaprism scanning method [3], and a real-time compensation scanning method, were used to perform wavefront one-dimensional path scanning. The calculation results are shown in Table 1 and Figure 5 shows the data fitting curves. The PV and RMS values measured by the traditional scanning method [3], the scanning method [2], and the real-time compensation method proposed in this paper, were 2.65 \(\lambda\) and 0.712 \(\lambda\), 1.208 \(\lambda\) and 0.360 \(\lambda\), and 0.512 \(\lambda\) and 0.115 \(\lambda\), respectively. The wavefront curve after de-tilting and defocusing was generally consistent with the nominal value distribution. The calculations show that the residual for the PV value was 0.072 \(\lambda\), and the residual for RMS was 0.041 \(\lambda\). It can be seen that the real-time compensation pentaprism scanning wavefront detection method isolated the guide rail errors and effectively improved the measurement accuracy. During the experiment, if non-parallel or non-perpendicular alignments occurred with regard to the components, a large error was introduced. The positions between the various components were adjusted, and the linear guide rail was moved as the spot on the CCD was observed. As the displacement distance of the guide rail increased, the spot collected by the CCD remained stable, and then the positional adjustment of the compensation device was complete. The compensated CCD had a large spot variation when the sampling was carried out. The centroid coordinate error could cause the compensated result to be larger than the pre-compensated result. When constructing the compensation device, the laser was ensured to be normally incident to the hypotenuse surface of the right-angle prism and to the CCD. In the experiment, the effect of the CCD focusing accuracy could be adjusted by observing the sharpness of the spot image. However, there is always uncertainty inherent to a human observer. Therefore, a set of scanning experiments were designed in this study. In order to represent the test results of the measured wavefront under different defocusing conditions, a number of defocusing positions were generated near the ideal focus. One-dimensional scanning along the sagittal direction of the emergent wavefront was performed near the actual defocusing point with 0.1 mm as the minimum distance step. The data obtained are shown in Figure 6, which correspond to the focal positions of 99.7 mm, 99.8 mm, 99.9 mm, 100 mm, 100.1 mm, 100.2 mm, and 100.3 mm. The results of the analysis indicated that when the lens focal length was 99.9 mm, the residual of the fitted curve for the emergent wavefront in the sagittal direction, as well as the residual for the actual wavefront, were the smallest, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Traditional** & **Double** & **Real-Time Compensation** & **Standard Values** \\ \hline PV/\(\lambda\) & 2.65 & 1.208 & 0.512 & 0.440 \\ RMS/\(\lambda\) & 0.712 & 0.360 & 0.115 & 0.074 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of experimental results. Figure 5: Fitting results with different methods. with a PV value of 0.512 \(\lambda\) and a residual of 0.072 \(\lambda\), respectively. Significant defocusing errors occurred in several other focusing positions.