Form measurement of slightly curved surfaces with diameters up to 1.5 meters based on an optical subaperture method

PTB is currently developing and setting up a new form measurement system for large optical surfaces with diameters up to 1.5 m. One objective is to measure flat surfaces with standard uncertainties of a few ten nanometers. Another objective is the form measurement of slightly curved specimens with a radius of curvature of 10 m or more with standard uncertainties below 1 μm. The new system will measure the specimen's topography in subapertures. Information is needed about the position and angle of each subaperture, so that the measurement errors of individual subapertures measurements do not lead to larger errors in the complete topography. For the measurement of slightly curved specimens, a highly accurate measurement of the rotations of the measuring head within a measuring range of ±5° is required. For this purpose, an angle measuring system for the measurement of the roll, pitch and yaw angle with an interference-based illumination is being developed at PTB.