Experimental study on subaperture testing with iterative stitching algorithm

College of Mechatronic Engineering and Automation, National University of Defense Technology, Changsha, Hunan, PR China.
Optics Express (Impact Factor: 3.49). 04/2008; 16(7):4760-5. DOI: 10.1364/OE.16.004760
Source: PubMed


Applying the iterative stitching algorithm, we demonstrate the power of subaperture testing through experiments. Naturally the algorithm applies to flats, spherical or aspheric surfaces. We first apply it to a silicon carbide flat mirror with larger aperture than the interferometer's. The testing results help to obtain a high-precision mirror through five iterations of ion beam figuring. The second experiment is 37-subaperture testing of a large spherical mirror. Good consistence is observed between the stitching result and the full aperture test result using a Zygo interferometer. Finally we study the applicability of the algorithm to subaperture testing of a parabolic surface. The stitching result is consistent with the auto-collimation test result. Furthermore, the surface is tested with annular subapertures and also retrieved by our algorithm successfully.

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    ABSTRACT: The measurement of aspheric surfaces in a Fizeau interferometer implies a sometimes dramatic increase in dynamic range, in terms of acceptable slope and departure, which can run the risk of introducing substantial measurement errors. Common approaches to relaxing the dynamic range requirement include reducing the area of the surface measured in a single measurement and stitching together the partial results, or using compensation techniques with the help of addi- tional components like null-lenses or computer generated holograms. This paper reviews these methods, with special attention to the questions of degrees of freedom for misalignment. These considerations lead to a proposed method that uses the inherent symmetry of the problem to scan along the optical axis, gathering measurements at zones of normal incidence. These measurements are independent from each other; their ensemble represents directly the surface-deviation in normal direction to the surface and the result is in the object coordinates of the design surface. Using an absolutely calibrated spherical reference surface, the result is absolute. It is shown that this is very different from the technique of stitching of zones, even when Intrinsic Coma is preserved through partially overlapping measurement regions.
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    ABSTRACT: A stereovision-assisted sub-aperture stitching (SAS) approach is proposed to solve the positioning problem of tested optics. The principle of stitching is analyzed and the overlapping calculation problem is simplified. Then, a binocular stereovision system is integrated into a test configuration. By measuring the coordinates of marks attached to the fixture in different positions, the stereovision system obtains the position and pose data of the optics to provide the initial values for optimization algorithm. The key benefit of stereovision is that the SAS test is easily performed without a precision positioning system. A simulation and practical examples are given to demonstrate the performance of this method. The stitching result shows good agreement with the full-aperture result.
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