Design and verification of an off-axis four-mirror freeform telescope for visible imaging

With the advancement of freeform optical design, fabrication, measurement, and assembly technology, the application of freeform optical systems is becoming increasingly widespread. The number of optical components in freeform optical systems has increased from two mirrors to three mirrors and four mirrors, and their application bands have also expanded from long-wave infrared (LWIR) to visible light. This paper designed and developed an off-axis four-mirror freeform telescope for visible light imaging. First, the optical and structural design is carried out according to the specified optical indicators. Then, an opto-mechanical-thermal coupling analysis is performed to verify the athermalization characteristics of the all-aluminum structure. Finally, through optical and mechanical alignment, the RMS of the center field of view of the telescope reaches 0.097λ (λ=632.8nm). Concurrently, thermal imaging experiments were conducted to obtain its resolution plate imaging at 65°C, 40°C, 16°C, and 0°C, with no significant changes observed in the images. Subsequently, the RMS of the telescope center field of view was retested after the thermal imaging experiment, showing a difference of 0.102λ, which demonstrates that the off-axis four-mirror freeform visible light telescope possesses excellent athermalization characteristics and imaging quality.