Shanshan Wang

SCOTS: a reverse Hartmann test with high dynamic range for GiantMagellan Telescope primary mirror segments

A software configurable optical test system (SCOTS) based on fringe reflection was implemented for measuring the primary mirror segments of the Giant Magellan Telescope (GMT). The system uses modulated fringe patterns on an LCD monitor as the source, and captures data with a CCD camera and calibrated imaging optics. The large dynamic range of SCOTS provides good measurement of regions with large slopes that cannot be captured reliably with interferometry. So the principal value of the SCOTS test for GMT is to provide accurate measurements that extend clear to the edge of the glass, even while the figure is in a rough state of figure, where the slopes are still high. Accurate calibration of the geometry and the mapping also enable the SCOTS test to achieve accuracy that is comparable measurement accuracy to the interferometric null test for the small- and middle- spatial scale errors in the GMT mirror.

Measuring rough optical surfaces using scanning long-wave optical test system. 1. Principle and implementation

Current metrology tools have limitations when measuring rough aspherical surfaces with 1-2 μm root mean square roughness; thus, the surface cannot be shaped accurately by grinding. To improve the accuracy of grinding, the scanning long-wave optical test system (SLOTS) has been developed to measure rough aspherical surfaces quickly and accurately with high spatial resolution and low cost. It is a long-wave infrared deflectometry device consisting of a heated metal ribbon and an uncooled thermal imaging camera. A slope repeatability of 13.6 μrad and a root-mean-square surface accuracy of 31 nm have been achieved in the measurements of two 4 inch spherical surfaces. The shape of a rough surface ground with 44 μm grits was also measured, and the result matches that from a laser tracker measurement. With further calibration, SLOTS promises to provide robust guidance through the grinding of aspherics.

Numerical simulation and experimental verification of extended source interferometer

Extended source interferometer, compared with the classical point source interferometer, can suppress coherent noise of environment and system, decrease dust scattering effects and reduce high-frequency error of reference surface. Numerical simulation and experimental verification of extended source interferometer are discussed in this paper. In order to provide guidance for the experiment, the modeling of the extended source interferometer is realized by using optical design software Zemax. Matlab codes are programmed to rectify the field parameters of the optical system automatically and get a series of interferometric data conveniently. The communication technique of DDE (Dynamic Data Exchange) was used to connect Zemax and Matlab. Then the visibility of interference fringes can be calculated through adding the collected interferometric data. Combined with the simulation, the experimental platform of the extended source interferometer was established, which consists of an extended source, interference cavity and image collection system. The decrease of high-frequency error of reference surface and coherent noise of the environment is verified. The relation between the spatial coherence and the size, shape, intensity distribution of the extended source is also verified through the analysis of the visibility of interference fringes. The simulation result is in line with the result given by real extended source interferometer. Simulation result shows that the model can simulate the actual optical interference of the extended source interferometer quite well. Therefore, the simulation platform can be used to guide the experiment of interferometer which is based on various extended sources.

Collimation testing using slit Fresnel diffraction

A simple collimation testing method based on slit Fresnel diffraction is proposed. The method needs only a CMOS and a slit with no requirement in dimensional accuracy. The light beam to be tested diffracts across the slit and forms a Fresnel diffraction pattern received by CMOS. After analysis, the defocusing amount and the distance between the primary peak point and secondary peak point of diffraction pattern fulfill an expression relationship and then the defocusing amount can be deduced from the expression. The method is applied to both the coherent beam and partially coherent beam, and these two beams are emitted from a laser and light-emitting diode (LED) with a spectrum width of about 50 nm in this paper. Simulations show that the wide spectrum of LED has the effect of smooth filtering to provide higher accuracy. Experiments show that the LED with a spectrum width of about 50 nm has a lower limitation error than the laser and can achieve up to 58.1601 μm with focal length 200 mm and slit width 15 mm.

Research on environment correction algorithm in the minimum deviation angle method for refractive index measuring

This paper studies environment correction algorithm in the minimum deviation angle method for refractive index measuring. The principle equation of minimum deviation angle method, based on the refractive index of air and the absolute refractive index of glass specimens is derived. The environmental factors are analyzed which may affect the measurement results in the process of actual measurement. According to thermal characteristics equations of glass, absolute index of refraction of glass for certain material is related to temperature. According to the Edlén equation, refractive index of air is related to temperature, pressure, humidity and so on. Sometimes, the environmental factors are uncontrollable, refractive index will change over the environmental factors, including temperature, pressure and humidity. The correction algorithm of refractive index which modified the measurement results from the non-standard environmental conditions to standard conditions is perfected. It improves the correction accuracy. Taking H-ZK9B for example, the impact of environmental factors on the refractive index is analyzed adopting controlling variable method. The need for environmental factors correction in different accuracy requirements is given. To verify the correction method, two sets of measured refractive index data of the same glass are corrected which measured under different environmental factors. The difference between the two sets of data is less than 1×10-6 with the correction.

Research on key technology in the real-time and high-precision spot centroid detection

This paper analyzes the influence on the centroid detection accuracy by several parameters, including the signal-to-noise ratio, frame rate and spot diameter. It provides the selection basis for the camera used in the centroid detection system. The diameter of the spot has little influence on the centroid detection accuracy within a wide range. Meanwhile, with the same signal-to-noise ratio, when the frame rate of the camera becomes higher, the centroid detection accuracy can be improved through the method of superposing more frames. The measurement software uses the specific multi-frame superposition denoising algorithm based on high speed CMOS camera which solves the conflict of improvement of accuracy and time-consuming of the algorithm. The repeatability of the centroid can reach up to 0.0023 pixel with the measuring speed of 62.5fps, the same as the frame rate of the camera.

Validation of reverse Hartmann test for mirror shape measurement of parabolic trough concentrator

A method for the surface shape qualification of a parabolic trough solar concentrator, based on the reflection of sinusoidal fringes in the mirror surface and their distortions due to the surface deviations from ideal geometry, has been developed. Without complex calibration, accurate surface slope data were collected and reduced by using sixteen-step phase shifting methods. As the gradient data may not be ideally available for the entire surface, surface deviations and geometrical parameters were obtained through Zernike slope polynomials iterative fitting and the Southwell integration algorithm. Additionally, an absolute reference measurement for calibration of the measurement accuracy using three standard mirrors is presented. It is shown that the proposed method can easily test the surface shape and concentration efficiency of a solar concentrator with high slope measurement accuracy (less than 0.05 mrad in the x-direction) and high spatial resolution (more than 2.5 × 106 points per mirror facet).

Real-time diameter measurement using diffuse light

A method for on-line rapid determination of the diameter of metallic cylinder is introduced in this paper. Under the radiation of diffuse light, there is a bright area close to the margin of metallic cylinder, and the method of this paper is based on the intensity distribution of the bright area. In this paper, with the radiation by a diffuse plane light with special shape, we present the relation expression of the distance between the peak point and the real edge of the cylinder and the distance between the diffuse light and the pinhole aperture of the camera. With the expression, the diameter of the cylinder to be measured can be calculated. In the experiments, monochromatic LED uniting with ground glass forms the diffuse light source, then the light irradiates the tested cylinder. After the cylinder, we use a lens with a front pinhole stop to choose the light into CMOS, then a computer is used to analyze images and export the measurement results. The measuring system using this method is very easily implemented, so it can realize the on-line rapid measurement. Experimental results are presented for six metallic cylinders with the diameter in 5~18mm range and roughness in Ra- 0.02um, and the precision reaches 3um.

Design of real-time and high-precision photoelectric autocollimator

This paper presents the design of a real-time and high-precision photoelectric autocollimator. Firstly, a far exit pupil distance and no vignetting optical system is proposed. Compared with the conventional optical system of autocollimator, it solves the problem of ray cutting which results in the detection accuracy changed with the distance of target lens. Besiedes, it eliminates the influence of stray light. Then, the design of photoelectric detection system is discussed. It uses the specific multi-frame superposition denoising algorithm based on high speed CMOS camera which solves the conflict of improvement of accuracy and time-consuming of the algorithm. Finally, test results of the instrument are given. The measurement repeatability of the autocollimator can reach up to 0.01″ with the measurement range of 1200″ and the single measurement time is less than 17ms.

Fizeau simultaneous phase-shifting interferometry based on extended source

Coaxial Fizeau simultaneous phase-shifting interferometer plays an important role in many fields for its characteristics of long optical path, miniaturization, and elimination of reference surface high-frequency error. Based on the matching of coherence between extended source and interferometer, orthogonal polarization reference wave and measurement wave can be obtained by Fizeau interferometry with Michelson interferometer preposed. Through matching spatial coherence length between preposed interferometer and primary interferometer, high contrast interference fringes can be obtained and additional interference fringes can be eliminated. Thus, the problem of separation of measurement and reference surface in the common optical path Fizeau interferometer is solved. Numerical simulation and principle experiment is conducted to verify the feasibility of extended source interferometer. Simulation platform is established by using the communication technique of DDE (dynamic data exchange) to connect Zemax and Matlab. The modeling of the extended source interferometer is realized by using Zemax. Matlab codes are programmed to automatically rectify the field parameters of the optical system and conveniently calculate the visibility of interference fringes. Combined with the simulation, the experimental platform of the extended source interferometer is established. After experimental research on the influence law of scattering screen granularity to interference fringes, the granularity of scattering screen is determined. Based on the simulation platform and experimental platform, the impacts on phase measurement accuracy of the imaging system aberration and collimation system aberration of the interferometer are analyzed. Compared the visibility relation curves between experimental measurement and simulation result, the experimental result is in line with the theoretical result.