Machining multiple mirror surfaces on one common substrate during the fabrication of off-axis three-mirror or four-mirror optical systems can take less time and drastically improve the alignment efficiency. However, the difficulty of the surface test remains the same. We theoretically propose a subaperture test method to carry out the null test of two mirrors on the synthetic reflective mirror. Specifically, we design a special zoom null lens and selectively use its subaperture wavefront aberrations of different configurations to nullify the surface normal wavefront aberrations of the according mirrors on the synthetic reflective mirror. The proposed method is verified by simulating the null test process of a synthetic reflective mirror integrating an off-axis high-order primary mirror (PM) and a coaxial high-order tertiary mirror (TM) of one off-axis three-mirror system, with the consideration of the fabrication and alignment errors. Simulation results show that, within a limited range of feasible tolerances, the residual wavefront aberration is 0.033λ root mean square (RMS) for the PM and 0.025λ RMS for the TM, at a wavelength of 1064 nm.
The null test of off-axis mirror plays a very important role in off-axis three mirror optical system design. The detection device depends on either complex detection optical system designs or high-precision null lens design. Both can only test one off-axis mirror and are not non-universal. In order to increase the detection efficiency, we innovatively propose a simultaneous null test method of primary mirror and three mirror in off-axis three mirror optical system. The simultaneous null test device is composed of a liquid zoom null lens. Simulation results show that the residual wavefront errors of testing a primary mirror and tertiary mirror are approximately 0.001λ peak-to-valley (PV), and 0.003λ (PV), respectively.
A method based on Seidel aberration theory to design a four mirror optical system with wide field of view was mentioned in this paper. In this method, Seidel aberration theory was studied, and the technical parameters of the system were got from the demands, then the technical parameters, such as the diameter of entrance pupil and the field of view were substituted into the Seidel aberration equations. Then we solved the equations with additional limit, and got the initial parameters of system, for example the radius of curvature of each mirror. The example in this article was a design of a four mirror system with a field of view of 15°and F number of 5. This example showed that the method based on Seidel aberration theory to design a four mirror optical system with wide field of view is effective and feasible.
The traditional zoom systems usually change the spaces between optical components to realize the variable focal lengths. Based on active optics, a new type of reflective active zoom system with four mirrors is proposed, which is different from the traditional zoom system by changing the spaces between optical components1. The new zoom system consists of an afocal front group with three mirrors, and the fourth mirror used for focusing. The relation among the four mirrors is determined by the Pezval condition. The secondary and third are deformable mirrors, contributing to the transition among different focal lengths by the curvature radius variation controlled by the voltage, with the constraints limiting its’ changing rules. According to the designed system characteristics and the practical requirements, based on 3rd-order aberration theory, a set of Seidel aberration coefficient functions are established, with the constraint limitations. Then the initial construction parameters of the optical system can be achieved. The new active zoom system with four mirrors can realize that the zoom rario is 3, the focal lengths vary from 100mm to 300mm, and the field angle of view range is 0.22° ～3.6°, and the wide working band is from the visible to the infrared. At the same time, because of the control flexibility, the active optical zoom system has the potential to be widely applied in remote sensing, life medical and other fields.
Reflective zoom system is widely used in the design of large size, wide spectral, high resolution system due to its great superiority in compacting size, system weight, aperture size, free chromatic aberration and thermo-stability. But for coaxial system, its disadvantage of obstruction renders the FOV (field of view) and light utilization rate unsatisfactory. Thus, to make the secondary and tertiary mirror off-axial is a good choice for optical designers. However, there are two problems in the alignment of off-axis zoom optical system. First, the Seidel aberration theory is not applicable for a system without rotational symmetry. Second, it is hard to control the misalignment status when zoomed. To solve this problem, the vector aberration theory is selected to analyze the off-axis three-mirror zoom system. When small perturbation is applied to the system, coincident with the alignment in reality, the residual aberration varies along with the movement of secondary and tertiary mirror. As the result, aberration character of misalignment three-mirror zoom system is provided, which offers guidance for misalignment determination and makes sure of the normal operation of the zoom system. This paper makes description of our experiment on an off-axis three-mirror zoom system and furnishes the figure of sensitivity in different zoom position. The conclusion may provide a reference to the vector aberration theory study on off-axis three-mirror zoom system and computer aided alignment.