KEYWORDS: Mirrors, Atomic force microscopy, Error analysis, Space mirrors, Data modeling, Optical components, Interferometers, Mathematical modeling, Zernike polynomials, Space operations
Zero-gravity surface figure testing is important for space optical elements. We propose the three orientations method (TOM) to achieve this goal. The optical element is rotated according to the proposed degree of freedom, and we calculate its zero-gravity surface figure by any three rotation angles (θ1, θ2, and θ3) and their corresponding surface figures (S1, S2, and S3). The algorithm of TOM can also be applied to the face-up/face-down test and horizontal rotation test. Theoretically, TOM can get an absolute zero-gravity surface figure; however, a rotation angle error can result in a zero-gravity surface figure error. The surface figure model (SFM) and gravity-included angle model (GAM) are proposed to analyze the relationship between the rotation angle error and zero-gravity surface figure error. For numerical results, SFM is equivalent to GAM. In addition, GAM addresses the zero-gravity surface figure error, which is induced by the rotation angle error, also being regarded as a surface figure, which is caused by a certain amount of gravity. For all models, we provide both theoretical derivations and numerical examples.
We present a method for extracting the zero-gravity surface figure of a mirror. If the structures and constraints of the mirror and its support mounts are invariable during measurement, we can use an algorithm to extract its zero-gravity surface figure by measured data obtained through three different orientations of the mirror. These orientations can be user-defined within the proposed rules, thereby reducing measuring constraints. The proposed algorithm is also available for the gravity-reversing method. To devise this algorithm, we propose and prove a nonlinear superposition property that the mirror system’s displacement of nodes at any gravity-included angle is equal to the sum of the projection of the displacement of nodes at θ = 0 and θ = π / 2. This method is verified through comparisons with a series of numerical stimulations.
The thermal induced effect errors including the surface distortion of heated mirror and micro-thermal turbulence fluctuation at the optical surface dramatically degrade the image quality of the telescope. To address the problem, we have proposed an air-knife system consisting of an annular flushing subsystem and a central sucking subsystem and reported its simulation analysis. This paper presents the detailed experimental performance of the air-knife thermal control system. The scaling experiment is conducted in a thermo-cycling experiment room with different environmental conditions, where the temperature fluctuations and wavefront perturbation of the scaling mirror can be accurately measured. It is shown from the experimental results that the approximately laminar forced air flow at the optical surface does blow away the turbulence fluctuation and not induce novel low order wavefront aberrations. Meanwhile, the air knife system contributes to the stability of the thermal boundary layer and enhances the convective heat exchange between mirror and air around. As a result, the air-knife system significantly decreases the surface-to-air temperature difference and improves the image quality with a thermal response. Furthermore, it is found that thermal control efficiency is less significant with the increase of the air intake flow or the decrease of the surface-to-air temperature difference. The scaling experiment results demonstrate the practicability of the air-knife thermal control system for large-aperture primary mirror.
Optical window working under differential pressure condition could deform and deteriorate optical quality. Therefore, an optical window manufactured and measured under zero differential pressure would induce transmission wavefront aberration, when it works under differential pressure condition. A new processing technology requires the optical window to be manufactured and measured under a given differential pressure condition which is equal to the actual working differential pressure of the optical window. The loop of manufacturing and measuring goes on, until the final measuring result satisfies requirement. The completed optical window could repeat measuring result when it works at the same differential pressure condition.
Optical system performance is easily affected by various surrounding conditions, especially the precision optical system, such as interferometer, goniometer, are sensitive to temperature fluctuations. Temperature is an important parameter of the air refractive index. Its fluctuation causes unsteady changes in air density and hence index of refraction changes. So the temperature of the detection room must be controlled in reasonable scope. But the heat released from devices, staff members and luminaires is unavoidable. This study calculates fluid field variations induced by temperature, obtains the air density and temperature distribution by Computational Fluid Dynamics (CFD) software, and then the fluctuations of refractive index are calculated from the density field. The optical path difference (OPD) is then given with density changes, also the OPD RMS values are computed. These data can quantify the influence degree of temperature changes and improve optical design.
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