Off-axis three-mirror optical system is an advanced optical design, through the combination of three mirrors, high-resolution and wide-field imaging. The system is widely used in the fields of astronomical observation, military reconnaissance and scientific experiment. In this paper, the structure and characteristics of off-axis three-mirror optical system are introduced to help readers understand its basic principles and advantages.
An off-axis three-mirror optical system usually consists of three main mirrors: primary mirror (M1) , secondary mirror (m2) and tertiary mirror (m3) . The combination of these mirrors forms the optical path of the system, and by reasonable design, high-efficiency optical performance can be achieved. The initial structure of the three mirrors I briefly analyzed in the last article, if you want to further study, you can refer to the“Optical aspheric design, processing and testing” book [2] .
On the basis of the initial structure obtained by coaxial calculation, we also need to design these mirrors off-axis, that is, they are not actually located on the optical axis, thus avoiding the central occlusion problem, the light flux and imaging quality of the optical system are improved.
The off-axis three-mirror optical system has the following notable features:
Off-axis design avoids the problem of shading the center part of the optical system, thus increasing the amount of light through the system. At Fourier’s optical angle, non-central occlusion also avoids the MTF curve of the optical system in the IF drop, which is beneficial to improve the imaging quality and target detection sensitivity.
High resolution and wide field of view: if all three surfaces are aspheric or even free-form, due to the large number of variables that can be used through precise optical design, the off-axis three-mirror system can achieve high resolution and wide field of view, which is suitable for the application of large-area observation, especially for the design of push-broom camera in remote sensing field. Of course, it should be noted here that the field of view of off-axis three-mirror optical system is often biased, and does not have rotational symmetry, and is sometimes restricted by the application of the scene.
Wavelength and environmental versatility: as a pure reflective optical system, does not produce chromatic aberration, so the off-axis tri-mirror system is also suitable for use as a multi-band detection system or with dual-color detectors. In addition, the non-thermal nature of the optical system is to correct the defocusing of the optical system in different environments, when the temperature changes, the system as a whole will be proportional to the thermal expansion and contraction, without thermal difference, which is also a significant advantage.
Flexible structure design: Off-axis design allows the optical system to have a greater degree of freedom, can be adjusted and optimized according to different application needs. However, the flexibility of this kind of structure design also brings difficulty to the design of optical system. At present, a frontier research direction is to combine the design of optical system and artificial intelligence to simplify the previous work as much as possible
The four types of structures are distinguished according to the folding direction of the mirrors and the relative positions between the mirrors, which have the following characteristics.
Tier 1 design: has maximum potential for free-form surface correction. In this design, the relative positions and angles between the mirrors are carefully designed to minimize the initial aberrations.
Tier 2 design: has some calibration potential, but is limited by volume. The Tier 2 design compromises certain geometric parameters relative to the Tier 1 design, resulting in slightly lower performance.
Tier 3 design: Limited calibration potential. This design has a large aberration in the initial structure, even if the free-form surface is used to correct, the final performance is not ideal.
Tier 4 design: minimum correction potential.
The initial aberration of this design is large, and the correction effect after using free-form surface is still not good enough to meet the requirements of high-performance imaging.
