## 4 Alignment Experiment  
After confirming the effectiveness of the design scheme in cophasing error adjustment by simulation, this scheme is applied to the actual Golay3 sparse-aperture imaging system. The fact that the three sub-light paths have their own optic axes is the biggest difficulty  
Figure 14: Comparison of optical paths with 0 tilt error and \(\alpha\) tilt error. Figure 15: Tilt error correction simulation. in the alignment. In order to ensure the accuracy requirements of the system, the entire alignment process can be divided into three steps: alignment of the single-beam _FSM_ structure, confocality of the three-beam steering structure, and correction of the cophasing error of the system. During alignment, manual rough adjustment is used to match with the _PZT_ precision adjustment. The alignment principle and process are shown in Figure 16a,b. After repeated iteration until the interference detection results meet the theoretical design requirements, the alignment results of the three light paths of the beam steering are shown in Figure 16c. In the light path where the subaperture 1 is located: RMS = 0.08\(\lambda\), and PV = 0.55\(\lambda\). In the light path where the subaperture 2 is located: RMS = 0.10\(\lambda\), and PV = 0.67\(\lambda\). In the light path where the subaperture 3 is located: RMS = 0.08\(\lambda\), and PV = 0.54\(\lambda\) (\(\lambda\) = 633 nm). The wave aberration RMS is superior to \(\lambda/8\) for all three subapertures, and the alignment results meet the theoretical design requirements. After the beam alignment is determined through the wave aberration analysis with the interferometer, the next step is to make the three subaperture beams converge to a point on the CCD through the synthetic imaging system by coarse adjustment, so that the three beams can be confocal. Through rough adjustment of the cophasing error, the piston error is controlled within 12 \(\upmu\)m and the tilt error is controlled within 1200 \(\upmu\)rad. After the alignment detection is performed for the subapertures, synthetic imaging system, beam steering, and adjustment structure, respectively, the three subapertures and the synthetic imaging system are installed on the connecting plate of the beam steering and adjustment structure. The system is installed on a pitchable 2D rotating frame. The physical prototype of the entire sparse-aperture imaging system is assembled, as shown in Figure 17a. The infinite point target is simulated using a large-diameter collimation, and the CCD image surface interference diagram is used as the detection object [20]. Rough adjustment is made for the _FSM_ mechanical structure of the three beams, so that the light spots of subaperture 1, subaperture 2, and subaperture 3 can interfere with each other. The interference pattern is shown in Figure 17b. Figure 16: Interferometric testing: (**a**) the principle, (**b**) the alignment process, and (**c**) alignment results displayed in the interferometer through _PZT_ precision adjustment. Since the field of view of the subaperture is 0.2\({}^{\circ}\), the center distance of the three subapertures is 320 mm. When the object distance is infinite, the fields of view of the three subapertures nearly coincide exactly with each other. When the object distance is finite, the field of view of the three subapertures can only overlap locally. The USAF1951 resolution test panel display 70 m away from the system is used as the surface target. The superimposed image of the field of view of the three subapertures is shown in Figure 17a. This synthetic image contains relatively large piston errors and tilt errors. By referring to the image of subaperture 1 in CCD, the _PZT_s of subaperture 2 and subaperture 3 are adjusted, respectively, and the piston error and tilt error of subaperture 2 and subaperture 3 are corrected. The synthetic image obtained after correction of the cophasing error is shown in Figure 18b.