Lidar metrology for prescription characterization and alignment of large mirrors

B. Eegholm, W. Eichhorn, R. von Handorf, J. Hayden, R. Ohl, G. Wenzel, José Sasián, Richard N. Youngworth
2011 Optical System Alignment, Tolerancing, and Verification V  
We describe the use of LIDAR, or "laser radar," (LR) as a fast, accurate, and non-contact tool for the measurement of the radius of curvature (RoC) of large mirrors. We report the results of a demonstration of this concept using a commercial laser radar system. We measured the RoC of a 1.4m x 1m spherical mirror with a nominal RoC of 4.6 m with a manufacturing tolerance of 4600mm +/-6mm. The prescription of the mirror is related to its role as ground support equipment used in the test of part
more » ... the James Webb Space Telescope (JWST). The RoC of such a large mirror is not easily measured without contacting the surface. From a position near the center of curvature of the mirror, the LIDAR scanned the mirror surface, sampling it with 1 point per 3.5 cm 2 . The measurement consisted of 3983 points and lasted only a few minutes. The laser radar uses the LIDAR signal to provide range, and encoder information from angular azimuth and elevation rotation stages provide the spherical coordinates of a given point. A best-fit to a sphere of the measured points was performed. The resulting RoC was within 20 ppm of the nominal RoC, also showing good agreement with the results of a laser tracker-based, contact metrology. This paper also discusses parameters such as test alignment, scan density, and optical surface type, as well as future possible application for full prescription characterization of aspherical mirrors, including radius, conic, off-axis distance, and aperture. 1, tel. 301 286 4342, fax 301 286 0204, Outline  LIDAR (laser radar = LR) technology is demonstrated for use in measuring the radius of curvature of large mirrors  Advantages are  Faster, lower manpower costs,  Reduced tooling needs  Lower risk of hardware damage,  Non-contact, in-situ with fabrication equipment  One metrology solution for multiple stages of telescope manufacture and verification,  Better accuracy 3 SPIE, San Diego, August 21, 2011 Concept summary  Application of LIDAR ("laser radar," LR) technology to aid the measurement of the prescription and alignment of large mirrors  Prescription: Radius, conic, aperture dimensions, off-axis aperture location/orientation  Alignment: Six degree of freedom alignment to a mechanical coordinate system, alignment to test set during surface fabrication (grinding, polishing), alignment during system integration, coarse phasing of segmented mirror systems  Process improvement over CMM-or laser tracker-based metrology/alignment systems JWST GSE mirror and LR metrology system (SSDIF clean room, NASA GSFC) Laser radar and mechanical metrology of part of JWST structure (SSDIF clean room, NASA GSFC)
doi:10.1117/12.895012 fatcat:rim6iql7n5hf7ab2yaj322hzgq