Objective  Infrared optical imaging system has excellent environmental adaptability and has been widely used in the field of optical imaging in recent years. In order to make the structure compact, the intermediate image plane formed by the primary and secondary mirror system often retains a large number of primary spherical differences, and the tolerance requirements are strict and the precision of installation is high. At present, the use of infrared interferometer is not popular in scientific research institutions, the commonly used interferometer is visible wavelength interferometer. As a result, the alignment of this kind of optical system often rely on the traditional alignment method that improves the machining accuracy of the mechanical parts and uses the center deviation measuring instrument to assist the positioning, the alignment accuracy cannot be guaranteed. In order to improve the alignment accuracy of this kind of optical system, it is necessary to discuss the alignment method, so that the high-precision alignment of optical system was realized. In this paper, the optical alignment method of a near-infrared refraction catadioptric system with intermediate image plane imperfect imaging of 150 mm aperture and 450 mm focal length was studied.

  Methods  Based on the near-infrared catadioptric optical system correcting lens group lens material does not block the visible waveband. In order to improve alignment accuracy, the lens blooming of correcting lens group was specially designed (Fig.2), the purpose of using the visible laser interferometer to detect the wave aberration of the system with high precision is realized. Based on the univariate analysis of the offset sensitivity, the coupling and aberration characteristics of each offset dimension are analyzed (Fig.4-5). Based on this, the optical setup scheme of the secondary mirror and the relay mirror group is proposed. The process of alignment strategy (Fig.6) is developed and the actual alignment experiment is analyzed.

  Results and Discussions  Experiments show that the center field view wavefront error (RMS) of optical system reaches 0.104λ, F(−1,0) field view RMS is 0.310λ, and F(+1,0) field view RMS is 0.188λ, F(0,−1) field view RMS is 0.216λ, F(0,−1) field view RMS is 0.176λ (Fig.9). When the center field of view is close to diffraction imaging, the edge field of view has serious asymmetry, which is mainly shown as primary astigmatism asymmetry, resulting in poor image quality of the edge field of view (Tab.2). At this time, adjusting the misalignment of the secondary mirror cannot completely eliminate the asymmetry of the edge field of view, so the position of the secondary mirror and correcting mirror group need to be adjusted after several iterations in the process of alignment. After fine alignment of the entire optical system, the center field view RMS reaches 0.114λ, F(−1,0) field view RMS is 0.109λ, and F(+1,0) field view RMS is 0.103λ, F(0,−1) field view RMS is 0.109λ, F(0,−1) field view RMS is 0.110λ (Fig.10), the RMS value of the full field is close to the design value @632.8 nm, achieving the imaging quality requirements

  Conclusions  The optical installation method of the near-infrared refraction and reflection system with non-ideal imaging of the middle image plane is mainly studied. The lens blooming of the near-infrared relay lens group is designed through the laser interferometer application band of 632.8 nm, so that the near-infrared optical system can use the visible laser interferometer for high-precision detection alignment. The alignment of intermediate image plane of imperfect imaging near-infrared catadioptric system was studied. Based on the univariate analysis of offset sensitivity, the coupling characteristics of each offset dimension are analyzed. On this basis, the optical setup scheme of secondary mirror and correcting lens group is proposed. The RMS wavefront errors of each field of view of the optical system after installation and adjustment are roughly the same as the numerical values of the design simulation results, and the direction is the same, both of which meet the requirements of imaging quality. The research results can provide an effective reference for the installation of such near-infrared refraction and reflection optical systems.