This technology enables accurate calibration of a large Computer Generated Hologram (CGH) fabricated without great accuracy, such that the CGH still measures an aspheric surface to an excellent accuracy of a couple of nm rms. The goal is the creation of software for generating a calibration map, and the fabrication of a couple of 9-in. (≈22.5-cm)-diameter CGHs to experimentally verify the technology. Use of CGHs in testing aspheric surfaces provides many advantages, such as better imaging, lower mapping distortion, and much higher-quality substrates.
NASA’s Jet Propulsion Laboratory developed a custom cassette that can write general CGH patterns on a 9-in. (≈22.5-cm)-diameter and 12-mm-thick, high-quality substrate. The writing accuracy is limited because the stage movement axes are not orthogonal and the stage calibration is not accurate. In this innovation, a proposed grid of cross hairs is embedded in the CGH pattern. The cross hairs’ positions can then be accurately measured after the entire pattern is written. These cross hairs contain the writing error information that can be used to apply a correcting to the wavefront error produced by the non-perfect CGH.
Both ground and space telescopes employ aspheric mirrors. A particular example is the x-ray telescope where primary and secondary mirrors have nearly cylindrical surfaces. CGH, in combination with commercial interferometers, provides high-resolution and high-accuracy measurements of aspheric optical surfaces. The current state of the art of CGHs are made on 6-in. (≈15-cm)-square substrates, such as those for testing the primary segments of the James Webb Space Telescope (JWST). However, larger CGHs are always desired. A larger CGH enables testing of correspondingly larger convex and nearly cylindrical concave surfaces in one shot. Studies have shown that larger CGHs also offer better imaging of the surface under test, which improves the test system’s Instrument Transfer Function (ITF), an equivalent metric to an imaging system’s Modulation Transfer Function (MTF).