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From 2015, NSPO (National Space Organization) began to develop off-axis Korsch telescope system for next generation earth observation mission. The experimental Korsch telescope system is consist of five mirrors, including: (1) M1: 550mm diameter clear aperture concave primary mirror, (2) M2: Convex secondary mirror, (3) M3: Off-axis concave tertiary mirror with rectangular aperture, (4) FM1 & FM2: Two folding mirrors with rectangular aperture and flat surface. All the experimental mirrors are designed with lightweight structure and made of fused silica. Since early 2016, we collaborated with Taiwanese domestic company and manufactured all the mirrors for the experimental Korsch telescope. Moreover, we not only accomplished the assembly of M1 but also implemented the form error metrology technique to measure the surface error of M1 with high repeatability in 2017. Recently, in order to validate the structural and athermal design of opto-mechanical structures, several vibration tests and thermal experiments have been accomplished in 2018. The experimental data could not only help us to enhance the analysis accuracy of the finite element model but also benefit to our automated opto-mechanical design system. For next developed phase, the aspheric polishing and reflective coating will be achieved till 2021.

Large mirrors with lightweight structure, such as those used in the telescope system of astronomy or spaceborne applications, are susceptible to stress caused by fabrication process. Furthermore, both the residual stress and subsurface damage are critical for the lightweight tooling of glass materials. In order to figure out the stress distribution on glass substrate, the photoelastic method has been applied to not only the astronomical optics but also the industrial optics of semiconductor equipment. However, there are several influence factors in photoelasticity measurement, including the structure of mirror, fabrication process, and metrology technique. The above factors may affect to the retardation result of photoelasticity measurement and cause the error of stress calculation. Therefore, it is important to clarify the retardation difference contribution to the corresponding influence factors. In this study, we attempted to use photoelastic instrument to investigate the relationship between the photoelastic effect and stress of several kinds of lightweight mirrors. There are three different lightweight mirrors were adapted to the photoelasticity measurement, including: (1) GSO 12” Mirror with 12 inches diameter made by fused silica, (2) Primary Mirror (M1-B) of Formosat-5 with 450 mm diameter made by ZERODUR® , (3) Primary Mirror (M1) of Mircrosat with 380 mm diameter made by CLEARCERAM®-Z. The experimental results depict some obvious retardation differences caused by the certain influence factors and the details will be discussed below.

This article presents the opto-mechanical design of a primary mirror assembly of a ground-based telescope with optimization algorithm. The prototype of ground-based telescope – GSO RC16 with 16 inches diameter blank primary mirror had been manufactured in 2016. However, a telescope with a blank primary mirror is too heavy to carry on for the stargazer. Besides, deformations caused by temperature difference and gravity will do significant effect to the large aperture mirrors with high optical performance requirements. In order to reduce the weight and maintain the stiffness simultaneously, the lightweight design and mounting interface design are critical and important. There are four types of system architectures in this project, including (1) two types of lightweight mirror designs - honeycomb type segments and sector type segments; (2) two types of mounting interface designs - retainer type support and CFRP type support. The optimization results showed that (1) the lightweight ratio of the primary mirrors are greater than 70%; and (2) the PV value of the mirrors supported by optimal mounting interfaces with gravity effect as a tilt of about 45 degrees and ±20°C temperature difference effectively less than 1/4 λ.

This paper presents the finite element and wavefront error analysis with reverse engineering of the primary mirror of a small space telescope experimental model. The experimental space telescope with 280mm diameter primary mirror has been assembled and aligned in 2011, but the measured system optical performance and wavefront error did not achieve the goal. In order to find out the root causes, static structure finite element analysis (FEA) has been applied to analyze the structure model of the primary mirror assembly. Several assuming effects which may cause deformation of the primary mirror have been proposed, such as gravity effect, flexures bonding effect, thermal expansion effect, etc. According to each assuming effect, we establish a corresponding model and boundary condition setup, and the numerical model will be analyzed by finite element method (FEM) software and opto-mechanical analysis software to obtain numerical wavefront error and Zernike polynomials. Now new assumption of the flexures bonding effect is proposed, and we adopt reverse engineering to verify this effect. Finally, the numerically synthetic system wavefront error will be compared with measured system wavefront error of the telescope. By analyzing and realizing these deformation effects of the primary mirror, the opto-mechanical design and telescope assembly workmanship will be refined, and improve the telescope optical performance.

FORMOSAT-5 consists of a spacecraft bus and an electro-optical payload. The payload is an f/8 Cassegrain type telescope with 3.6-m effective focal length. The spacecraft has a ground sampling distance of 2-m for panchromatic and 4-m for multispectral bands, with a 24-km swath width. FORMOSAT-5 is the first space program that National Space Organization (NSPO) takes full responsibility for the complete satellite and payload system engineering. The optical system assembly (OSA) has been successfully aligned and is now undergoing final performance verification tests at system level. To help create this unique instrument, NSPO has developed the computer aided alignment method assisted with the mechanical ground support equipment to carry out the assembly, alignment, and verification of the complex systems. This method offers an integrated capability for interferometric alignment and characterization of the large instrument. A detail OSA integration and verification steps, including primary mirror, secondary mirror, corrector lens and baffles alignment are presented. This paper describes the overall capability of this method and uses decomposed Zernike polynomials from the alignment and characterization of the OSA to verify the reduction of the wavefront errors and misalignments. It further demonstrates the successful completion of the instrument and satisfaction with the main system requirements.

For a currently developing multispectral space Cassegrain telescope, the primary mirror with 450 mm clear aperture is made of Zerodur and lightweighted at a ratio about 50 % to meet both thermal and mass requirement. For this mirror, it is critical to reduce the astigmatism caused from the gravity effect, bonding process and the deformation from the mounting to the main structure of the telescope (main plate). In this article, the primary mirror alignment, MGSE, assembly process and the optical performance test for the primary mirror assembly are presented. The mechanical shim is the interface between the iso-static mount and main plate. It is used to compensate the manufacture errors of components and differences of local co-planarity errors to prevent the stress while iso-static mount (ISM) is screwed to main plate. After primary mirror assembly, an optical performance test method called bench test with novel algorithm is used to analyze the astigmatism caused from the gravity effect and the deformation from the mounting or supporter. In an effort to achieve the requirement for the tolerance in primary mirror assembly, the astigmatism caused from the gravity and deformation by the mounting force could be less than P-V 0.02λ at 633 nm. The consequence of these demonstrations indicates that the designed mechanical ground supported equipment (MGSE) for the alignment and assembly processes meet the critical requirements for primary mirror assembly of the telescope.

Optical parameters such as radius of curvature (RoC), direction of the optical axis, offset of the apex relative to the outer diameter, et al. of the primary mirror of a Cassegrain telescope by a coordinate measuring machine (CMM) is presented. These parameters are measured by a novel technique developed by the authors. RoC, tilt, and wedge of a lens can also be measured by the technique. Geometric parameters, such as diameters, central obscuration diameter, and perpendicularity of mirror edge, the mirror, et al. can be measured taking the advantage of the geometric measurement function. The optical and geometric parameters are measured by this method on a set of primary and secondary mirrors, and four corrector lenses of a Cassegrain telescope.

This paper presents the analysis of static structure and wavefront error (WFE) of a telescope experimental model’s primary mirror. The experimental telescope with 280mm diameter primary mirror had been assembled and aligned in 2011. The WFE result was not perfect. In order to find out the root cause of the WFE, the static structure analysis had been applied to the structure model of the telescope assembly. Some assumed effects which may cause deformation of the primary mirror had been proposed, such as gravity effect, iso-static mount (ISM) bonding effect, thermal expansion effect, etc. According to each assumed effect, we established a corresponding model and boundary setup. The numerical model was also analyzed by static structure analysis software and opto-mechanical analysis software to obtain numerical WFE and Zernike polynomials. The numerically synthetic optical system WFE was compared with measured system WFE of the telescope. According to the results of these deformation effects of the primary mirror, we could get the root cause of the system WFE. In the next generation of telescope, the opto-mechanical design and alignment process would be refined, and applied to the new optical system.