Structural Inspection of an Astronomical Wide Field Corrector with a Portable CMM

Abstract

Accurate assembly characterization is critical to the proper assembly and alignment of optical systems, such as the Hobby-Eberly Telescope wide field corrector (WFC). The WFC is comprised of four mirrors built into a steel structural housing, with a maximum weight of 1,800 pounds; the alignment of the mirrors depends on accurate knowledge of the structural dimensions. To examine the structure of the WFC system and find out the decentration, we used the FaroArm portable coordinate measuring machine combined with SpatialAnalyzer software. The next step was to decide how to adjust the system to make it well aligned based on the measurements. We found concentricity of the major portions of the assembly that was less than 0.5 mm. HET/WFC is mainly comprised of three concentrated mechanical parts holding four large mirrors, as seen in figure 1. The concentricity among these three parts is crucial to the optical performance, but there will be some decentration among these parts because of the errors induced in the manufacturing process. To examine the structure of the WFC system and find out the concentricity, we used a FaroArm combined with SpatialAnalyzer software to measure the structural features, calculate the angles and distances between each part, and then determine the concentricity of the system.