A short focal length telescope is essential for remote sensing instruments (RSI) to achieve sub-meter resolution, with the premise of precise fabrication and alignment. The tolerance becomes tighter when the focal length becomes shorter, which makes the assembly issue even harder. In this paper, the catadioptric telescope with short focal length is taken as the target for developing a design methodology with the consideration on the permissible tolerance specifications of mirror surface root-mean-square (RMS) wavefront error (WFE) in system tolerance for fabrication, as well as the tolerance of surface tilt and decenter of mirror in assembly process. The parameter chosen as the trade-off variable is the aperture size of the telescope, and the tolerance budget for aligning high-resolution RSI is evaluated upon the requirement of a ground sampling distance (GSD) less than 60 cm. The result has been used as the guideline for finding optimal aperture size in an experimental remote sensing satellite (RSS) project.
This article presented the design improvement based on the modal frequency shifting method for resolving the structural response excitation of a catadioptric telescope. For the high image quality demand optical system, the positioning stability of optics is crucial. Taking the catadioptric telescope for example, the most sensitive part is the position of secondary mirror within several micrometer tolerance along optical axis. Therefore, the opto-mechanical design for maintaining the positioning of optics while the system suffering from the harsh environmental vibration becomes important.
This paper describes superior candidate substrate material- aluminum nitride (AlN), for the cost-effective and high-performance
requirements for aerospace mirrors. In fact, high specific stiffness and thermal stability are not only two major
considerations to select ideal material but also machinability, dimensional stability, and cost. Silicon carbide shows the
best figure of merits 12627, but it has extremely low remove rate and expensive raw material properties. Extremely low
expansion ceramics such as Zerodur may be difficult to obtain in large quantities and its figure of merit is 1151. On the
contrary, AlN has outstanding thermal conductivity (~170 W/m°C) and maintain high figure of merit 2222 without
compromise. In our fabrication processes, the AlN substrate can be easily polished to 53±1nm Ra surface texture and
66.4±2nm RMS surface form. Its unique thermal stability, specific stiffness, and good figure of merits, associated with its
easy machinability and low-cost raw materials, makes AlN an ideal selection for superior mirror substrate in the future
space mirror optics.
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.
For the earth observation mission, there are some critical environmental requirements including low-light condition, fast moving objects, high scanning rate. In order to meet these requirements, the Time-Delay-and-Integration (TDI) technique is critical and essential for the sensor part to improve the Signal to Noise Ratio (SNR) performance. National space organization (NSPO) collaborates with National Chip Implementation Center (CIC) on the next generation image sensor. In order to increase SNR under the light-starved condition, a 32-stages digital-accumulator Time-Delay-and-Integration (TDI) CMOS image sensor is adapted to improve the image quality. Besides, it could successfully take several pictures under different TDI stages on a dynamic test bench. The experimental results verified that the 32-stage TDI CMOS image sensor could function well.
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.
In 2015, NSPO (National Space Organization) began to develop the sub-meter resolution optical remote sensing instrument of the next generation optical remote sensing satellite which follow-on to FORMOSAT-5. Upgraded from the Ritchey–Chrétien Cassegrain telescope optical system of FORMOSAT-5, the experimental optical system of the advanced optical remote sensing instrument was enhanced to an off-axis Korsch telescope optical system which consists of five mirrors. It contains: (1) M1: 550mm diameter aperture primary mirror, (2) M2: secondary mirror, (3) M3: off-axis tertiary mirror, (4) FM1 and FM2: two folding flat mirrors, for purpose of limiting the overall volume, reducing the mass, and providing a long focal length and excellent optical performance. By the end of 2015, we implemented several important techniques including optical system design, opto-mechanical design, FEM and multi-physics analysis and optimization system in order to do a preliminary study and begin to develop and design these large-size lightweight aspheric mirrors and flat mirrors. The lightweight mirror design and opto-mechanical interface design were completed in August 2016. We then manufactured and polished these experimental model mirrors in Taiwan; all five mirrors ware completed as spherical surfaces by the end of 2016. Aspheric figuring, assembling tests and optical alignment verification of these mirrors will be done with a Korsch telescope experimental structure model in 2018.
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.
For meeting the requirements of the high-precision telescopes, the design of collimator is essential. The diameter of the collimator should be larger than that of the target for the using of alignment. Special supporting structures are demanded to reduce the deformation of gravity and to control the surface deformation induced by the mounting force when inspecting large-aperture primary mirrors. By using finite element analysis, a ZERODUR® mirror of a diameter of 620 mm will be analyzed to obtain the deformation induced by the supporting structures. Zernike polynomials will also be adopted to fit the optical surface and separate corresponding aberrations. Through the studies under different boundary conditions and supporting positions of the inner ring, it is concluded that the optical performance will be excellent under a strong enough supporter.
Remote sensing instrument (RSI) is used to take images for ground surface observation, which will be exposed to high vacuum, high temperature difference, gravity, 15 g-force and random vibration conditions and other harsh environments during operation. While designing a RSI optical system, not only the optical quality but also the strength of mechanical structure we should be considered. As a result, an optimization method is adopted to solve this engineering problem. In the study, a ZERODUR® mirror with a diameter of 466 mm has been chosen as the model and the optimization has been executed by combining the computer-aided design, finite element analysis, and parameter optimization software. The optimization is aimed to obtain the most lightweight mirror with maintaining structural rigidity and good optical quality. Finally, the optimum optical mirror with a lightweight ratio of 0.55 is attained successfully.
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