Off-Axis Three-Mirror Reflective Optical System

1, Schiefspiegler optical system

A coaxial reflective optical system, suffers from aperture obstruction, which affects both the light-gathering ability and the diffraction resolution of the optical system. This phenomenon is particularly pronounced when the field of view is large. Off-axis reflective optical systems, on the other hand, do not have aperture obstruction and thus have higher light energy utilization for the same optical aperture, making it easier to achieve higher imaging quality. The history of off-axis reflective optical systems can be traced back to 1616 when Zucchi proposed the forward-looking telescope scheme. Zucchi placed a concave bronze mirror at an angle as the objective lens of the telescope. The light beam reflected by the bronze mirror was directly coupled to a refractive Galilean eyepiece with negative focal power, forming a visual astronomical telescope system. To avoid the observer’s head obstructing the bronze mirror, this telescope required the use of an off-axis field of view for observation. Given the design and manufacturing conditions of the time, the bronze mirror not only lacked detailed surface parameter design and had low surface accuracy, but also did not have the technological capability to coat high-reflectivity films on the mirror substrate. Therefore, it was impossible to achieve good imaging quality.

To eliminate the central obstruction in the Cassegrain system, Anton Kutter published an article in 1953 about the use of two tilted mirrors. In this two-mirror optical system, the primary and secondary mirrors are tilted at specific angles to remove the aperture obstruction caused by the secondary mirror on the primary mirror.

In his article, Kutter introduced the German word “Schiefspiegler,” which means “tilted mirror.” Since then, this term has become a representative general term for such unobstructed optical systems. Subsequently, various forms of Schiefspiegler two-mirror and three-mirror systems have emerged. The Buchroeder Schiefspiegler three-mirror system shown in Figure 14 and the Solano Schiefspiegler three-mirror system shown in Figure 15 are two examples of Schiefspiegler three-mirror systems.

However, due to the tilt of the mirrors introducing a significant amount of coma and astigmatism into the optical system, Schiefspiegler-type optical systems only achieve satisfactory imaging quality when the relative aperture is below 1:20. This limitation has restricted the promotion and application of Schiefspiegler optical systems.

Off-Axis TMA Optical System

Currently, one of the more widely used off-axis reflective optical systems is the off-axis Three-Mirror Anastigmat (TMA) optical system. Unlike the Schiefspiegler system, in the off-axis TMA system, the parent mirrors of the three reflective surfaces are aligned along a single axis, which serves as the optical axis of the system. The three reflective surfaces are sub-aperture regions of their respective parent mirrors. Therefore, the off-axis TMA reflective optical system is an offset sub-section of the coaxial TMA reflective optical system. From this perspective, one can understand the transformation process from a coaxial TMA system structure to an off-axis TMA system structure. Below, two commonly used structural forms are illustrated as examples.

The first type is the field-offset off-axis TMA optical system. This system is formed by offsetting the field of view based on the coaxial TMA optical system. A common type uses the secondary mirror as the aperture stop, and the primary and tertiary mirrors can also be simultaneously offset.

The second type is the aperture-offset off-axis TMA optical system. This system is formed by offsetting the aperture based on the coaxial TMA optical system. A common type uses the primary mirror as the aperture stop, and the optical system’s field of view can also be simultaneously offset. The reflective surfaces in off-axis TMA optical systems are typically conic sections or higher-order aspheric surfaces. With the advancement of optical technology, freeform optical surfaces are also being gradually applied and have promising application prospects. As the imaging field of view increases, the optical system needs to address not only the correction of the primary three monochromatic aberrations but also the design of a flat field. Therefore, in off-axis TMA optical systems, at least one mirror must have negative optical power to correct field curvature. Korsch summarized 10 types of off-axis TMA optical system structures that can achieve flat-field design, as shown in Figure 17. Some of the optical systems introduced later in the text also fall within these structural types.

As can be seen from the figure, similar to the coaxial three-mirror reflective optical system, the off-axis TMA optical system can also be broadly divided into two categories: systems without an intermediate image plane, i.e., single-imaging off-axis TMA optical systems, and systems with an intermediate image plane, i.e., double-imaging off-axis TMA optical systems. The structural forms of these two types of off-axis TMA optical systems are very diverse. Cook has applied for many patents in this area and has seen considerable application, so many off-axis TMA optical systems are also named Cook-type optical systems.