The high-accuracy phase description of the coherent light propagation is important to assess and optimize far-field and astigmatic optical systems, such as interferometric surface test, etc. Usually, the wavefront propagation is simulated by the physical optics theory throughout each optical element, which might be time-consuming and incur computational complexity. In this article, we analyze the wavefront degeneration in the CGH interferometric null test by the complex ray tracing. We sample the wavefront by a series of elliptical Gaussian beamlets, which is consistent with the Eula formula in the differential geometry. The propagation of elliptical Gaussian beamlets through the optical system can be calculated by the tiny ray bundle theory and geometric optics. In the output space, the wavefront can be described by an elliptical Gaussian beam originated from the propagated Gaussian waist through the system. Therefore, the phase distribution at the output space can be characterized by the sum of series of elliptical Gaussian beams, which gives a high-accuracy analytical simulation of phase distribution better than 30mrad with time about 0.2s. As an example, we apply our method to the analysis of the wavefront degeneration in the interferometric null test of a ⌀3m aspheric mirror. After the optimization, the instrumental transfer function increased from 0 to 0.65 at 0.4 Nyquist frequency.
An illumination system for coherent noise suppression is envisioned. The principle of coherent noise suppression is introduced, and this illumination system is designed this way. The illumination system is simulated, and the hybrid sequence model of Zemax analyzes the irradiance. To verify the noise suppression effect of this illumination system, it is used as the illumination part of the Fizeau interferometer. The interference process is simulated under the nonsequential model of Zemax to obtain an off-axis light source with a ring radius of 0.32 mm and a numerical aperture of 0.14. The Fizeau interferometer with a conventional light source is also simulated. A comparison experiment is set up to generate the same noise point in the two interferometers using different illumination modes to trace and produce the same four interference fringes with the same interferometric cavity length of 20 mm and the same tilt angle of the measured surface of 0.057°. Compared with the interferograms in the conventional illumination mode, the interference fringes formed by the illumination of this study are almost undamaged, and the near-complete interference information can be retained. The interferometer system with this light source was built and the test results were verified, and it was found that it could achieve the measurement accuracy of 1/20 wavelength and the measurement stability of 1.219 nm, and it also had a good contrast of interference fringe.
As the large-scale SiC materials, CCOS and MRF processing technology is gradually developing, optical manufacturing quickly entered the "era of giant mirror". As every basis of all optical processing, surface testing technology is facing enormous challenges with high efficiency and high precision and high accuracy. As an essential method in large aperture optical testing, we proposed a worthy attempt to estimate the actual surface error map RMS from individual measurement results. In order to overcome some actual working condition limits, it is a supplement to the state-of-the-art large sample averaging methods. This part of work is based on the orthogonality hypothesis, and proposed the real surface error estimation methods using interact scalar product. The experiment of a ∅3m aspherical mirror measurement and analysis indicates that the actual surface error RMS is 0.020~0.023λ, with estimation accuracy ±λ/700.
Lightweight optics utilized in high-power laser system can dissipate heat better and reduce system weight. Dielectric films can be designed for laser pulse modulation, high reflect rate and wide bandwidth. However, the deformation caused by coating residual stress will be large due to the large thickness of dielectric films and low stiffness of lightweight mirror, which brings a large bending deformation with the temperature variation. The traditional surface compensation technology is double-sided coating, but it is not suitable for lightweight mirrors. In this study, we construct a finite shell-element model to estimate the surface shape error caused by dielectric film coating residual stress on a lightweight mirror for surface pre-processing. Thermal deformation and bimetal effect deformation are also simulated.
Epoxy adhesives are widely used in opto-mechanical joints for large optics. Much work has been done for bonding, fabricating and testing TMT M3, which is to be fabricated in CIOMP. General bonding procedures are introduced, specimen testing is taken for strength values, lower strength limit are predicted for application safety and stability. Based on these work, a 1/4 scaled prototype of the M3 and two dangerous optics at 1.5 m level for a large Fizeau interferometer to test M3 are successfully bonded, with some details given about the locating precision of the support pads. Results showed lower strength limits of tensile 51/shear 70 MPa, namely safety factors of 6 and 17 for the two optics respectively, considering stress concentration. Additionally, nominal strength limits of tensile 6. 7/shear 7.0 MPa at 99.95% reliability were expected. The team is in good readiness for M3 production.
M3M (Mirror 3 Mirror) of TMT (Thirty Meter Telescope) project is a 3.5m×2.5m×0.1m solid flat elliptical mirror. CIOMP is responsible for the fabrication of M3M as part of China’s share in TMT project. CIOMP have beaning working on this project with TMT Corporation since 2012. The requirement of M3M surface including plate scale (18.8 mas), SlopeRMS (0.8μrad) and roughness (Rq 2.2nm). The testing and fabrication technologies required by M3M are challenges for CIOMP especially 6 years ago. To date, Most of Fabrication and testing technology of M3M have been developed and verified at CIOMP. CIOMP has passed reviews including CoDR, PDR, FRR and TRR successfully with board invited by TMT Corporation. Technologies developed for M3M are including on cite large aperture vertical Fizeau sub-aperture interfere test, scanning pentaprism system, and polishing technology with the goal of minimum slopeRMS. A 1/4 equal-stiffness prototype is also fabricated serving as a pathfinder for the fabrication of M3M. All requirement for M3M are achieved on M3MP successfully. This paper give a brief introduction of the work on M3M, M3MP and some of our results. This work is also supported by Major international cooperation projects of the National Natural Science Foundation.
Ion beam figuring (IBF) is an advanced and deterministic method for optical mirror surface processing. The removal function of IBF varies with the different incident angles of ion beam. Therefore, for the curved surface especially the highly steep one, the Ion Beam Source (IBS) should be equipped with 5-axis machining capability to remove the material along the normal direction of the mirror surface, so as to ensure the stability of the removal function. Based on the 3-RPS parallel mechanism and two dimensional displacement platform, a new type of 5-axis hybrid machine tool for IBF is presented. With the hybrid machine tool, the figuring process of a highly steep fused silica spherical mirror is introduced. The R/# of the mirror is 0.96 and the aperture is 104mm. The figuring result shows that, PV value of the mirror surface error is converged from 121.1nm to32.3nm, and RMS value 23.6nm to 3.4nm.
A feasible way to improve the manufacturing efficiency of large reaction-bonded silicon carbide optics is to increase the processing accuracy in the ground stage before polishing, which requires high accuracy metrology. A swing arm profilometer (SAP) has been used to measure large optics during the ground stage. A method has been developed for improving the measurement accuracy of SAP using a capacitive probe and implementing calibrations. The experimental result compared with the interferometer test shows the accuracy of 0.068 μm in root-mean-square (RMS) and maps in 37 low-order Zernike terms show accuracy of 0.048 μm RMS, which shows a powerful capability to provide a major input in high-precision grinding.
M3M (Mirror 3 Mirror) of TMT (Thirty Meter Telescope) project is a 3.5m×2.5m×0.1m solid flat elliptical mirror. M3MP is a 1/4 prototype of M3M serving as a pathfinder for M3M. Fabrication and testing of M3MP were carried out based on planned sketch for M3M established in the past 2 years. Technology including polishing strategy, on site vertical Fizeau sub-aperture interfere test, scanning pentaprism system and dual-supporting system were tested in the fabrication of M3MP. This paper give a brief introduction of the work on M3MP and some of results.
For polishing the ultra-thin TMT M3MP, a polishing support system with 18 hydraulic supports (HS) is introduced. This
work focuses on the designing and testing of these HSs. Firstly the design concept of HS system is discussed; then
mechanical implementation of the HS structure is carried out, with special consideration of fluid cycling, work
pressurization and the weight component. Afterward the piping installation and the de-gas process for the working fluid
are implemented. Pressurization and stiffness are well checked before system integration for the single HS unit. Finally
the support system is integrated for the polishing process.
Large flat mirrors play important roles in large aperture telescopes. However, they also introduce unpredictable problems. The surface errors created during manufacturing, testing, and supporting are all combined during measurement, thus making understanding difficult for diagnosis and treatment. Examining a high diameter-to-thickness ratio flat mirror, TMT M3MP, and its unexpected deformation during processing, we proposed a strain model of subsurface damage to explain the observed phenomenon. We designed a set of experiment, and checked the validity of our diagnosis. On that basis, we theoretical predicted the trend of this strain and its scale effect on Zerodur®, and checked the validity on another piece experimentally. This work guided the grinding-polishing process of M3MP, and will be used as reference for M3M processing as well.
The PSS (pentaprism scanning system) has advantages of simple structure, needless of reference flat, be able of on-site testing, etc, it plays an important role in large flat reflective mirror’s manufacturing, especially the high accuracy testing of low order aberrations. The PSS system measures directly the slope information of the tested flat surface. Aimed at the unique requirement of M3MP, which is the prototype mirror of the tertiary mirror in TMT (Thirty Meter Telescope) project, this paper analyzed the slope distribution of low order aberrations, power and astigmatism, which is very important in the manufacturing process of M3MP. Then the sample route lines of PSS are reorganized and new data process algorism is implemented. All this work is done to improve PSS’s measure sensitivity of power and astigmatism, for guiding the manufacturing process of M3MP.
The swing arm profilometer (SAP) has been playing a very important role in testing large aspheric optics. As one of most significant error sources that affects the test accuracy, misalignment error leads to low-order errors such as aspherical aberrations and coma apart from power. In order to analyze the effect of misalignment errors, the relation between alignment parameters and test results of axisymmetric optics is presented. Analytical solutions of SAP system errors from tested mirror misalignment, arm length L deviation, tilt-angle θ deviation, air-table spin error, and air-table misalignment are derived, respectively; and misalignment tolerance is given to guide surface measurement. In addition, experiments on a 2-m diameter parabolic mirror are demonstrated to verify the model; according to the error budget, we achieve the SAP test for low-order errors except power with accuracy of 0.1 μm root-mean-square.
SiC mirrors are of excellent mechanical and thermal properties compared to their glass opponents. Efficient fabrication and testing of SiC mirrors, particularly the large freeform surfaces are very challenging. In this paper, direct CNC generation, deterministic polishing techniques including CCOS, MRF, and IBF were addressed in detail. Since testing is critical to make high accuracy freeform surfaces, the paper focused on Computer Generated Hologram (CGH) design and implement to measure large mirrors. In particular, detector to mirror mapping distortion was discussed in detail. Finally, some results for SiC mirrors were presented.
Three Mirror Anastigmat (TMA) systems including both on-axis and off-axis configurations have been widely used in space applications. In some designs, to correct for high order aberrations and realize large FOV, freeform surfaces are used to provide more design freedoms. This trend brings challenges to optical manufacturing and testing community. Since testing is critical to make high accurate aspheres and freeform surfaces, the paper addressed Computer Generated Hologram (CGH) design and implement to measure large freeform mirrors. In particular, CGH assisted alignment procedure for TMA telescopes were discussed in detail.
In this paper, the influence on the image quality of manufacturing residual errors was studied. By analyzing the statistical distribution characteristics of the residual errors and their effects on PSF and MTF, we divided those errors into low, middle and high frequency domains using the unit “cycles per aperture”. Two types of mid-frequency errors, algorithm intrinsic and tool path induced were analyzed. Control methods in current deterministic polishing process, such as MRF or IBF were presented.
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