UlE, as a kind of ultra-low thermal expansion glass, has been widely applied in large-aperture optical mirrors and space telescopes. However, hard-brittle material feature of ULE brings certain difficulty for machining. In this paper, rotary ultrasonic vibration-assisted peripheral grinding (RUPG), combining rotary ultrasonic vibratio (UV) and conventional grinding (CG) is proposed, and grinding process and quality characteristic are investigated in terms of surface morphology, grinding force, surface roughness, subsurface damage depth, and subsurface morphology. A serious of comparative experiments for between RUPG and CG were conducted. The results show that rotary UV in RUPG can markedly decrease the grinding force with a factor of 46.78%. The change rule of grinding force with the varying of grinding parameters in RUPG is consistent with that in CG, i.e., grinding force increases as the increasing of grinding depth and feed rate, while it decreases with the growth of spindle speed. Grinding surface marks were found to be obviously weakened by UV and surface consistency was accordingly improved. Besides, measurement results of surface roughness also real that UV plays a positive role in diminishing surface roughness by almost 23.01%, and reducing the subsurface damage depth by a factor of 17.19%.
A variable curvature mirror (VCM) fabricated by 3D printing technique which is thickness optimized in structure design to reduce spherical aberration and supposed to be used in zoom imaging system is investigated. First, measurement and parameters fix of the mirror blank printed by 3D printing of AlSi10Mg are done for its precision deviation introduce by the manufacturing method. Second, elementary optical polishing is done for the purpose of Nickel plated. Fine optical polishing is applied on the VCM after the Nickel plated. Third, an actuation test experiment is built and tested by piezoelectric actuators of PI with nanometer precision and Zygo interferometer. The original surface figure accuracy of 90% radius is 2.225 λ / 0.394 λ (λ = 632.8 nm). As a result, within the ultimate testing range of the interferometer, the VCM achieve about 8.68μm deformation with the corresponding position change of actuator is 18μm, which is about 50% of it. Finally, an experiment of zoom imaging effect is done. The experiment shows that it does have effect to the zoom imaging which can compensate the defocus within 230.7μm. From the performance of the VCM at this stage, it can be used in infrared imaging. For the following work, its structure will be further optimized and the precision problems will be solved through using more proper manufacture method to improve its radius change performance during actuation process. Therefore, it can be used in visible light imaging in the future.
To realize ultra-lightweight ratio, mirrors were fabricated with CFRP (carbon fiber reinforced plastic composites) by replication technology. The replication technology was aimed to improve surface accuracy. Though replication technique, the surface accuracy was improved to 0.098 λ (λ=632.8nm, RMS), and the roughness of 1.9 nm (Ra) can be achieved. The CFRP mirror presented poor dimension stability, the surface accuracy increased gradually in air. In order to solve this problem, a polymer coating was carried on mirror surface. The polymer coating exhibited better dimension stability, the surface accuracy can be maintained under 0.15 λ for more than 200 days.
To improve the reflecting properties of all CFRP mirror, a high reflecting coating must be prepared on the mirror surface. In this paper, the effect of the roughness, film material and the deposition temperature on the reflecting rate was discussed. In the experiments, it was observed that the film exhibited higher reflecting rate on the smoother surface; meanwhile, the deposition rate must be controlled below the soften point of the surface replicated resin; if not, pits will generate on the surface and reduce reflecting rate. Ag film system exhibited higher reflecting rate than Al films. Finally, a multilayer film Ag and SiO2 was deposited on CFRP mirror, with a reflecting rate over 95% between 450nm and 800 nm.
Due to low density, high specific stiffness, and low thermal expansion, carbon fiber reinforced plastic (CFRP) is believed to be one of the potential material choices for optical mirrors. But CFRP is one of the two-phase materials that cannot be used as optical surface and must be surface modified. To develop one kind of grid-reinforced CFRP mirror, optical replication technology was used to modify and achieve high-precision surface, and theoretical deformation due to replica resin curing and deformation caused by laminates’ manufacturing errors were studied in detail. Optical replication experiment has shown that λ / 20 root mean square high-precision surface can be achieved for ϕ100-mm grid-reinforced carbon fiber mirrors.
The advantage of Carbon Fiber Reinforced Polymer (CFRP) is obvious as a common space material for low density, low thermal expansion coefficient and high specific stiffness characteristics, it is the ideal material choice for space optical reflector. Mirror structure with honeycomb can achieve high rates of lightweight, as well as high specific stiffness. For Φ300mm CFRP mirror, accounting of the actual process properties of CFRP, mirror panels laminated based on thermal stability design, honeycomb fabricated using one innovative inlaying-grafting design method. Finally, lightweight structure design of the CFRP primary mirror completed, the thermal stability result of the Φ300mm CFRP mirror achieved is 10nm°C.
Up to now, traditional materials, such as glass, metal and SiC ceramic, gradually begin to be unsatisfied development of
the future mirrors. Designable carbon fiber reinforced composites became optimized material for large aperture
lightweight mirrors. Carbon/carbon composites exhibit low thermal expansion and no moisture-absorption expansion
problem, therefore, they get particular attention in the space reflector field. Ni was always employed as optical layer in
the mirror, however, the coating behaved poor bond with substrate and often peeled off during optical processing. In
order to solve this problem, slight oxidation was carried on the C/C composites before Ni plated. The Ni coating
exhibited stronger coherence and better finish performance. Finally, a 100mm diameter plane mirror was successful
fabricated.
Due to low density, high stiffness, low thermal expansion coefficient, duplicate molding, etc., carbon fiber reinforced polymer (CFRP) is one of the potential materials of the optical mirror. The process developed for Φ300mm high precision CFRP mirror described in this paper. A placement tool used to improve laying accuracy up to ± 0.1°.A special reinforced cell structure designed to increase rigidity and thermal stability. Optical replication process adopted for surface modification of the carbon fiber composite mirror blank. Finally, surface accuracy RMS of Φ300mm CFRP mirror is 0.22μm, surface roughness Ra is about 2nm, and the thermal stability can achieve 13nm /°C from the test result. The research content is of some reference value in the infrared as well as visible light applications.
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