Objective  When the aspheric primary mirror and fourth-mirror integrated molding manufacturing method is used in coaxial four-mirror optical system, the complexity of system parts and weight of the whole machine would be reduced, and the installation efficiency could be improved greatly. Besides, the degree of freedom is constrained in later optical system assembly, so the optical axis of the two aspheric mirror needs maintain a high degree of consistency in the mirror manufacturing process. On the basis of the existing interferometry method, a new method is proposed to measure the optical axis deviation of two-sided community aspheric mirror.

  Methods  Based on the existing interferometric measurement method, a method of calculating the optical axis consistency of CGH interferometric wavefront tilt is proposed. The principle of the measurement method is introduced in detail (Fig.1). Figure 1 (a) shows the CGH optical measurement system of the two-sided community aspheric mirror. The surface of the aspheric mirror to be measured is S1 and S2. Interferometer 1 and CGH1 are used to measure the surface shape of the aspheric surface S1. The optical axes of CGH1 and aspheric S1 will reach a high consistency after the primary aberration is controlled strictly in the interferometric measurement optical system. Similarly, the optical axes of CGH2 and aspheric S2 would also be consistent. The optical axes of CGH1 and CGH2 would respectively characterize the optical axes of S1 and S2. The optical axes of CGH1 and CGH2 are both perpendicular to their optical surfaces in the design model. Figure 1 (b) shows the interferometric optical system for measuring the angle between two CGHs. CGH1 is designed to emit parallel laser in a specific area, and after reflection by CGH2, an interference fringe representing the angle between two CGH compensators is formed in the interferometer. The optical axis deviation of the two aspheric surfaces can be obtained by solving wavefront tilt of the interference fringe (Eq.1).

  Results and Discussions  For a diameter Φ500 mm two-sided aspherical mirror, the optical measurement model was designed and simulated, the design parameters were given (Tab.1-2). The diffraction stray light in the measurement optical path was simulated and analyzed (Fig.3). The error sources affecting the measurement accuracy (Fig.4-5) were decomposed. The main error sources are CGH manufacturing error, optical path misalignment error, and angle measurement error between CGH1 and CGH2. Simulation analysis shows that the measurement accuracy is 1 s. Two CGH were designed and processed, and the interference measurement optical system was built (Fig.6). The optical axis angle was calculated as (0.007 0°, 0.002 0°) when the wavefront tilt was (1.544λ, 0.441λ). The angle between the two CGH remeasured by theodolite was (0.007 1°, 0.001 9°). The profiler method was used to compare and verify CGH measurement result, the surface contours of the primary mirror and the four mirrors was scaned respectively, and the optical axis deviation was (0.007 1°, 0.002 0°) after unifying in one coordinate system.

  Conclusions  In order to solve the problem of optical axis consistency measurement of two-sided aspherical mirror, a new method of calculating CGH interference wavefront tilt was proposed based on the existing interferometric method. The principle of the method was introduced, the simulation design and error analysis of the measurement optical system were carried out, which show 1 s accuracy. The optical system was built and the method of profilometer was compared to verify the measurement accuracy of the method. This method has the advantages of intuitiveness and high measurement accuracy, and has been successfully applied to the integrated molding manufacturing of coaxial four-trans optical system.