Large Aspheric Mirrors Research Articles

Intelligent splicing technology of off-axis aspheric mirrors based on Zygo interferometer

With the increasing size of the aspheric, manufacturing and testing technologies face enormous challenges. A Zygo interferometer-based intelligent splicing technology for off-axis aspherical mirrors is proposed in this paper, which provides a "small to big" manufacturing strategy for large aspherical mirrors. In the proposed method, a Zygo interferometer, a splicing adjustment platform, and a computer-integrated algorithm are combined to achieve closed-loop adjustment of the off-axis aspherical mirrors. Taking any one of the off-axis aspherical sub-mirrors as a reference, the point cloud information on the surfaces of all off-axis sub-mirrors is obtained by combining the optimal spherical wavefront data with the Zygo interferometer data. The splicing adjustment platform performs positional corrections to the to-be-adjusted off-axis aspherical mirrors using the calculated point cloud registration parameters as the adjustment amount. The iterative operation of point cloud alignment and splicing platform adjustment is done until the positional deviation of the adjusted off-axis aspherical mirror surface reaches a predetermined threshold value, at which point the adjustment stops. The experimental verification employed two off-axis aspherical sub-mirrors with a 100 mm aperture that belonged to the same aspherical surface. The result shows that large aspherical mirrors can be made quickly and effectively with intelligent splicing technology.

New capabilities for laser holographic testing during assembly and collimation of large segmented telescope mirrors

Subject of study. Several novel options for interferometric testing of large aspheric segmented primary telescope mirrors during all phases of construction (assembly, collimation, and performance assessment) are proposed. In this paper, we will describe and validate new engineering design solutions for interferometric surface testing of large concave aspheric segmented telescope mirrors. Methods. All proposed test configurations are based on a reflecting optical compensator in the form of an axial synthetic holographic optical element or a reflective convex aspheric surface of revolution. We propose a quasi-autocollimative light path in which an optical compensator reverses the object wavefront; however, the object wavefront itself does not directly produce the image of the surface being tested in the plane where the interference and shadow patterns are recorded, thereby eliminating significant distortion-like errors. Main results. We provide the calculated parameters for quasi-autocollimative systems used to test the surfaces of the aspheric concave primary mirror segments of four well-known world-class telescopes: the Millimetron telescope, diameter 10 m; the James Webb Space Telescope, diameter 6.5 m; the Extremely Large Telescope (ELT), diameter 39.3 m; and the postponed European Southern Observatory conceptual design known as the Overwhelmingly Large Telescope (OLT), diameter 100 m. We show that simultaneous use of a series of similar coaxial optical compensators on the object leg of the interferometer will almost completely eliminate any restrictions related to the size, asphericity, and curvature of aspheric segments of primary mirrors in telescopes. Using a conical substrate for the working surface of the holographic compensator will substantially reduce the maximum spatial frequency of the compensator. The calculations were performed using Mathcad and Zemax. We propose that traditional non-autocollimative laser holographic test configurations without wavefront reversal be used on the object leg during the initial assembly of the primary mirror segments. Laser holographic topography is also suitable for this purpose. Practical significance. The novel ideas, methods, and systems engineering solutions proposed here are based on wavefront reversal by a reflecting optical compensator or a series (“chain”) of such compensators and will support in-process testing and certification testing of aspheric primary mirror segments, with interferometric accuracy, for any of the optical telescopes currently operating or under development, whether ground based or space based. These capabilities will support full-scale interferometric testing of the 12 m diameter Millimetron aspheric segmented primary mirror initially proposed by Academician N. S. Kardashev, but will also undoubtedly be especially important for space-based telescopes.

Simple and Efficient Non-Contact Method for Measuring the Surface of a Large Aspheric Mirror

A non-contact measurement method for measuring large aspheric surfaces with a laser tracker is proposed. Using an air-bearing probe eliminates the need to contact the optical surface and improves measurement efficiency and accuracy. Using this method, we measured the surface of an aspheric mirror 3 m in diameter and 13.6 m in the radius of curvature. The preliminary experimental result indicates that the error of surface measurement is 0.8 μm (RMS).

Creating a large aspheric primary mirror using spherical segments

The use of aspheric mirrors is a common practice to design astronomical telescopes with a few optical elements. In the most preferred optical design Ritchey Chretien (RC), both primary and secondary mirrors are hyperboloid. Nowadays large telescopes are being built using small mirror segments, however, making aspheric off-axis mirror segments is still a challenge. We have conducted a study in which, we explored the possibility to mimic an aspheric hyperbolic primary mirror by making use of smaller spherical mirror segments. Three different methods have been used to form a large segmented aspheric primary of nearly 12m aperture. In the first method, fixed ROC(radius of curvature) spherical mirror segments are reconfigured by a piston, tip, and tilt (PTT). In the other two methods, in addition to PTT, ROC of the segments are also varied. We further attempted to reduce the telescope wave-front error by varying the segment size and the F ratio of the primary. We found out that none of these three methods provided acceptable image quality unless we incorporate the warping harness in the segment support. The use of the warping harness emulated by Zernike coefficient correction, remarkably reduced the wave-front error and delivered a decent image quality over a large field of view. In this paper, we present the results of our study on designing an RC type optics for a 12m class optical-NIR(Near Infrared) telescope using spherical mirror segments.

A Methodology for Measuring the Parameters of Large Concave Aspherical Mirrors by Means of a Wavefront Sensor

A methodology has been developed for measuring radius Rv and eccentricity (conic parameter k) of large concave aspherical mirrors using a wavefront sensor. Analytical expressions that directly relate Zernike coefficients a4 and a9 to parameters Rv and k of the mirror are obtained. It is shown that the technique does not require accurate mirror alignment before measurements. A computer analysis showed that the developed scheme enables measurements with errors of δRv < 0.1% and δk < 0.01 for mirrors with radii from 100 to 2000 mm and with errors of δRv < 0.01% and δk < 0.001 for mirrors with radii of more than 5000 mm.

Measurement Vertex Position of a Large Aspheric Mirror

We present a novel method to accurately measure the vertex position of a large aspheric mirror with a phasing measuring interferometer, a laser tracker, and a micro alignment telescope. The method consists of rotating the mirror by 180° about its mechanical axis and measuring the change in displacement. Use of a micro alignment telescope eliminates the need to adjust the mirror during the measurement, eases the alignment of the testing system, and improves the vertex position measurement accuracy. Using this method, we measured the vertex position of an aspheric mirror 1 m in diameter and 2 m in radius of curvature. The vertex position measurement uncertainty is 88 μm.

Pose error correction in offline three-dimensional profile measurement system for large-diameter aspheric mirror.

This paper proposes an offline measurement method for a large aspheric mirror during its grinding stage. A measurement method and mathematical model to realize a three-dimensional (3D) profile measurement for large-diameter aspheric mirrors are proposed, based on a cylindrical coordinate system. The measurement error induced by the alignment error is examined based on the error analysis of the measurement and workpiece coordinate systems. The corresponding pose error obtained through the nonlinear least squares method is eliminated to improve the measurement accuracy. To evaluate the entire surface profile error, a 3D profile model is established. The total uncertainties of the proposed measurement system are also estimated. Comparison experiments for a 300-mm-diameter K9 glass workpiece are conducted on UPFM900 and Taylor Hobson PGI 3D profile instruments, and the profile accuracy is found to be improved from 15.3 to 7.12μm after eccentricity and tilt error elimination.

Bironchigram processing method for quantitative evaluation of optical focusing mirrors

Since its proposal the Ronchi “ruling” test has been considered as the most remarkable technique of the Schlieren family. Despite being based on fringe pattern generation and commonly interpreted in a qualitative way, it was capable of achieving outstanding results in the detection of aberrations in optical surfaces. Given its theoretical straightforwardness, experimental simplicity and economic affordability in comparison to other methodologies in the field, in the present work is addressed the lingering issue of how to extract quantitative information from digital captures of this test. Such task was accomplished taking advantage of the improved Ronchi test with a square grid, from which it was possible to develop a mathematical model with foundation in the Fourier theory to describe its observations. Thereupon, it was devised a novel iterative algorithm grounded in Fourier phase measuring able to compute fairly accurate wavefront gradient estimations from a single “bironchigram” sample with grid defocus. This procedure is complemented by a mandatory integration algorithm that is based on the regularization theory. The overall proposed methodology was put to test in a large aspherical mirror, estimating a surface aberration profile with a P–V (peak to valley) ratio of 155 nm, value well in accord with the one reported by the commercial interferometric system used for validation.

Tool setting error compensation in large aspherical mirror grinding

Increasing competition and high accuracy make it necessary to optimize and evaluate the outcome of manufacturing processes. In tool grinding, models for the tool path and tool setting are of particular interest. In this article, to establish a valid general grinding model, an improved method for tool setting error compensation is proposed. We consequently developed models of tool path and tool setting, and verified the models by simulations. Lastly, the model is demonstrated through testing of a concave aspheric surface. The experiment results show that the introduced models are valid for grinding in aspherical mirror, since they are solely based on the geometry and process parameters, and hence are applicable for manufacturing process optimization.

大口径非球面反射镜误差分离组合加工技术

大口径非球面反射镜误差分离组合加工技术

Large optical aspheric mirror profile data evaluation method based on Levenberg-Marquardt algorithm

Large optical aspheric mirror profile data evaluation method based on Levenberg-Marquardt algorithm

大口径批量化离轴非球面镜的检测光路设计

大口径批量化离轴非球面镜的检测光路设计

大口径离轴非球面补偿检测的畸变校正

大口径离轴非球面补偿检测的畸变校正

Promising methods for quality control test of large convex aspherical mirrors

Promising methods for quality control test of large convex aspherical mirrors

Research on the error analysis and compensation for the precision grinding of large aspheric mirror surface

The purpose of this paper is to propose a compensation grinding method for large aspheric mirror surface. Because the tradition grinding is not suitable for large aspheric mirror surface, in this paper, a grate parallel grinding in 3-axis and an on-machine measurement system are applied. Based on that, it presents the errors that mainly affect the form accuracy and technology of compensation grinding. An experiment of compensation grinding was carried out for large aspheric mirror surface. With the separation of decentering, wheel arc, and residual grinding system error, the PV value further decreased comparing to tradition compensation. These results indicated that this compensation can improve the form accuracy significantly in large aspheric mirror surface grinding.

대구경 비구면 광학기술과 응용

A large aspheric mirror is a key component for large astronomical telescopes and high resolution satellite cameras. Since it is large and has an aspheric form, it is much more difficult to fabricate it compared to the similar size of spherical mirror. Especially, the opto-mechanical design and analysis is critical to reduce the deformation of mirror surface due to the external forces such as gravity or temperature change, as the mirror size is larger and lightweighting ratio is increased. The design requirements for the mirror are different depending on the particular ground and space applications because the environmental conditions are changed. In this paper, we explain the opto-mechanical design and analysis for ground and space applications that are among the most difficult to achieve among several technologies related to development of the large aspheric mirror.

Research on the Normal Error of Large Aspheric Mirror Employed in Bonnet Polishing

The influence functions action orientation of the Bonnet Polishing (BP) process is perpendicular to the contact zone. So the normal error of the aspheric surface should be used as the residual error to calculate the dwell time. But the Z axis direction error is generally adopted as the residual error and few papers on the normal error have been reported hitherto. Its necessary to pay attention to this issue. In this paper, two algorithms which are Asphericity Subtraction (AS) algorithm and Z Axis Direction Error Transformation (ZADET) algorithm are presented to calculate the normal error of the large aspheric surface. Simulations in three different cases are organized to utilize these two algorithms, together with the comparison of them. And the comparison of the normal error and the Z axis direction error is also organized. Its found that there exists difference between AS algorithm and ZADET algorithm. Both of them can be used to calculate the normal error of the aspheric surface when the ratio value of the width to radius is small. And the difference between the normal error and Z axis direction error is considerable. So the normal error should be used as the residual error to calculate the dwell time in BP process.

大口径非球面的组合加工

To manufacture a large aspheric mirror in higher efficiency and higher precision,a new method by using multi-mode optimization is proposed to improve material removal efficiency and surface convergence rates.On the basis of matrix calculation,this method optimizes multi-processing cycle in one optimizing processing simultaneously and extends the calculating process of the dwell time for traditional Tool Influence Functions(TIF)to complete the integrating optimizing solution of multi TIF.As the solution area of dwell time is extended,the bigger grinding head and the small one have complementary advantages and obtain a global optimization for multi grinding heads and multi-processing cycles.In the end,an error map is simulated with actual parameters by using computer virtual multi-mode method.The simulation result shows that the multi-mode technique can improve the processing efficiency by 50%as compared with traditional method and its convergence rate(RMS value)is the same as that of the smaller grinding head.When the simulation is on the real surface,the RMS changes from 0.011 5to 0.004 4and the convergence rate is 0.621 2,which satisfies the requirements of real manufacture.The multi-mode technique improves polishing efficiency in maintaining good precision and is an attractive solution for the large optical aspheric mirror fabrication.

Computer-aided alignment in null testing of large aspheric mirrors

During the null testing of aspheric mirrors, the error induced by misalignment of the null compensator or the mirror will couple with the surface figure and consequently give incorrect information for deterministic figuring. A mathematical alignment model of null testing is established, and the computer-aided alignment software is developed to determine the misalignment in this paper. Based on the analysis of aberration compensation with different alignment parameters, numerical simulations show that high accuracy alignment of null testing can be achieved by adjusting only five degrees of freedom of the null compensator, on condition that a planar interferometer is used and the amount of misalignment is small.

Figuring of Aspherical Metal Mirror Substrate for Neutron Focusing by Numerically Controlled Electrochemical Machining

Neutron beam generated by high intensity proton accelerator facility is powerful tool to investigate characteristics of soft and hard materials. However, neutron beam is not major tool for material science since intensity of neutron beam is very weak compared to that of X-rays. Neutron focusing device is required to increase in intensity of neutron beam. Aspherical supermirror is effective for neutron focusing with wide wavelength range without chromatic aberration. In this research, we proposed a fabrication process for large and cost-effective aspherical mirror substrate made of aluminum alloy because metal can be figured coarsely at low cost by using conventional machining. The mirror fabrication process proposed by us consists of grinding for coarse figuring, numerically controlled electrochemical machining (NC-ECM) to correct objective shape with form accuracy of sub-micrometer level and low-pressure polishing to decrease in surface roughness to sub-nanometer level. In the case of figure correction of the mirror substrate by NC-ECM, deterministic correction is realized because NC-ECM is a non-contact electrochemical removal process for metal materials, without workpiece deformation. In this paper, we report fundamental machining characteristics of ECM, which uses electrode with a diameter of 10 mm and NaNO3 electrolyte.