We present a criterion to properly choose the ruling frequency during the testing process of concave mirrors using the classical Ronchi test. It is known that when the number of lines per inch (ruling frequency) is low, the Ronchi test loses sensitivity; this fact implies that it is not qualitatively possible to determine the real surface shape; only an approximation would be obtained. In addition, if a higher ruling frequency is used, the ronchigram would be exposed to diffractive effects, making it even more difficult to identify the patterns for the real surface shape. We have found that by mathematically relating the f-number of the surface and the ruling spacing, the detection range of the Ronchi test can be improved. This allows us to know the shape of the patterns with the best certainty corresponding to a given optical surface. We have analyzed the behavior of real ronchigrams produced for two different parabolic mirrors to demonstrate this fact. In addition, real ronchigrams obtained from primary mirrors of telescopes that support the use of this criterion are shown.
This work aims to design a filter to attenuate high- and medium- frequency noise in optical test images without changing
the edges and original characteristics of the test image, generated by traditional filters (spatial or frequential). The noise
produced by the LCD pixels (used as a diffraction grating in the Ronchi test) was analyzed. The diffraction is modulated
by the spherical wavefront of the mirror under test, generating at least two frequency band noise levels. To reduce this
bi-frequential noise, we propose to use an array of filters with the following structure: a low-pass frequential filter LPFF,
a band- pass frequential filter BPFF and a circular mask spatial filter CMSF; thus obtaining the composed filter
CF=LPFF-(BPFF)(CMSF). Various sizes of filters were used to compare their signal-to-noise ratio against simple filters
(low-pass and band-stop).
In this work we have developed a design fringe projection method to obtain profilometry of a suitable object by using Dammann gratings encoded into an LCD as a modulator space and a monochromatic light source. Dammann grating are diffraction binary gratings commonly used to generate diffraction points with the same intensity in Fourier plane. Diffraction order and spatial frequency are parameter of grating design, several gratings types can be designed and projected on objects by controlling parameters of design.
We present a new method for testing an optical surface. It uses the Ronchi test with variable-frequency rulings and a liquid-crystal display. The rulings can be formed by substructuring the spacing of a Ronchi ruling or combining several classical Ronchi rulings in a single variable-frequency ruling. This change allows us to observe smaller defects on the surface, because it enlarges the spatial-frequency domain of the ruling, and a larger dynamic range of detection of the Ronchi test can be obtained instead of increasing the resolution of the detection of the Ronchi test by iteratively changing classical Ronchi rulings with higher line density. As a result, we have found that it is possible to measure defects on a optical surface that are of size 57 nm (/11).
We show an optimization method based on an evolutive algorithm to obtain the profile of a simulated machined aspherical surface starting from a set of noisy discrete Cartesian coordinates (x,y,z), where the experimental coordinates x and y are used to simulate the sagitta z of the analyzed surface. By minimizing an objective function, the proposed method fits the sagitta function to the set of noisy discrete coordinates; thus the geometrical parameters of the simulated surface under test, such as the paraxial radius of curvature, the conic constant, and the aspheric deformation constants, can be obtained. Numerical results show that our method can be successfully applied to retrieve the simulated machined surface profile.
In order to overcome classic polishing techniques, a novel hydrodynamic radial polishing tool (HyDRa) is presented; it is useful for the corrective lapping and fine polishing of diverse materials by means of a low-cost abrasive flux and a hydrostatic suspension system that avoids contact of the tool with the working surface. This tool enables the work on flat or curved surfaces of currently up to two and a half meters in diameter. It has the advantage of avoiding fallen edges during the polishing process as well as reducing tool wear out and deformation. The functioning principle is based on the generation of a high-velocity, high-pressure, abrasive emulsion flux with radial geometry. The polishing process is repeatable by means of the control of the tool operational parameters, achieving high degrees of precision and accuracy on optical and semiconductor surfaces, with removal rates of up to 9 mm3/hour and promising excellent surface polishing qualities. An additional advantage of this new tool is the possibility to perform interferometric measurements during the polishing process without the need of dismounting the working surface. A series of advantages of this method, numerical simulations and experimental results are described.
HyDRa is a hydrodynamic radial polishing tool ideal for the corrective lapping and fine polishing of diverse materials by means of an accelerated abrasive flux. The roughness of an optical surface is analysed for a continuous manufacturing process, beginning with the basic generation steps up to a finished optical surface. These results were obtained using a Linnik interferometer.
We propose a method for piston errors detection in a segmented surface by means of the classical ronchi test. This consists in comparing the ronchigram fringes frequency of a reference and piston segment. The comparison is developed for the correlation method of the intensity vs pixels curves of the reference and piston segment. The presence of the piston term in a ronchigram is assured experimentally for the shack interferometer, it is by observing the coincidence rings centered type fizeau in each segment. The proposed method is applied to a segmented spherical surface with a two segment mirrors, and resolutions of piston ⩾63 ηm are experimentally obtained.
We propose one method for the phase alignment of segmented mirrors with piston errors. For this, we use the classical Ronchi test to observe fringes around of curvature center, after we take some lines on the Ronchi image for each segment mirror and obtain its intensity vs. pixels, to do the correlation and approximate the amount of pixels or piston displacement for the phase misalignment. Additionally, we get parabolic fit to find resolutions of sub-pixels. At the same time, we use the Shack Interferometer to observe behavior rings centered for this two segment mirrors to compare with the fringes produced with Ronchi test and to achieve resolutions of piston >= 24 (eta) m.
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