Asphercial Metrology for Non-Specular Surfaces with the Scanning Long-Wave Optical Test System
AdvisorBurge, James H.
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PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractAspherical optics are increasingly used these days. The application of aspherical surfaces on large, astronomical telescope mirrors brings challenge to the fabrication. Since the surface radius of curvature varies across the surface, the grinding/polishing tool needs to change its shape when working on different parts of the surface, making surface error more easily embedded into the surface. Therefore, a tighter test-fab loop is needed to guide the fabrication process. To maximize the accuracy during the grinding of the surface and to minimize the working time in the polishing stage, a better metrology device that can measure rough surface is needed to guide the grinding process. Scanning long-wave optical test system (SLOTS) is designed to meet this demand by providing accurate, fast, large dynamic range, and high spatial resolution measurements on rough optical surfaces (surface rms roughness<1.7 µm).SLOTS is a slope measuring deflectometry system that works like a reversed wire test. It measures the reflection of the infrared light off the test surface, and calculates the local slope of the test surface. The surface sag/height is obtained through integration. During the test, a heated metal ribbon radiates long-wave infrared light that is reflected by the test surface. A thermal imaging camera records the reflected light. The ribbon is scanned in two orthogonal directions. From the variation of the irradiance recorded by the camera, slope maps of the test surface can be retrieved in the two orthogonal directions. SLOTS is a combination of tradition slope measurement and modern technology, processing advantages from both parts. It measures surface slope, so there is no need for null optics. It uses an uncooled thermal imaging camera that is made with high resolution and high sensitivity. The linear stage used to scan the hot ribbon has long travel, small resolution, and high accuracy. Both the camera and stage enable SLOTS a large dynamic range and high sensitivity. SLOTS has successfully guided the grinding process of the primary mirror of Daniel K. Inouye Solar Telescope. This mirror is a 4-meter diameter off-axis parabola (OAP). Its largest aspherical departure is 8 mm. SLOTS is able to measure it without any null optics. Under the guidance of SLOTS, the surface shape was controlled to be 1 µm rms within designed shape (with astigmatism removed) at 0.7 µm rms surface roughness (12 µm loose abrasive grits).
Degree ProgramGraduate College