Variable optical null based on a yawing CGH for measuring steep acylindrical surface

Measurement of steep acylindrical surface has the difficulty with respect to its large localized slope, which may lead to irresolvable fringe densities in off-axis subapertures. To address this problem, we analyze the departure of off-axis acylindrical subapertures, and propose a measurement strategy by yawing the cylinder null. When the cylinder null is yawed with different angles, variable mounts of acylindrical wavefronts are generated to compensate most of the aberrations for different off-axis subapertures. Thus, the fringe densities are drastically reduced within the vertical dynamic range of interferometers. To connect all subaperture together, we also propose an acylindrical stitching approach. Experimental results demonstrate that an acylindrical lens with a departure of up to 81µm from the best-fitting cylinder can be measured using the proposed method. More importantly, it does not require an additional reconfigurable optical null, making the measurement system simple and inexpensive.