Very large computer generated holograms for precision metrology of aspheric optical surfaces

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. Computer Generated Holograms (CGH), in combination with commercial interferometers, provide high resolution and high accuracy measurements of aspheric optical surfaces. The current state of the art CGHs are made on 6" square substrates such as those for testing the primary segments of James Webb Space Telescope. 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 CGH null test system's Instrument Transfer Function, an equivalent metric to an imaging system's Modulation Transfer Function. Furthermore, pursuit of improving CGH test accuracy never ends. The fundamental limiting factor is quality of the substrate. The current state of the art technology is still unable to fabricate general CGHs of arbitrary symmetry on high quality custom substrates. Arizona Optical Metrology LLC (AOM) proposes to address these problems. We propose to work with our collaborators at the ebeam facility of Jet Propulsion Laboratory of NASA to write large CGHs on high quality substrates. We anticipate the writing has reasonable yet non-negligible errors which cause errors in the aspheric wavefront the CGH produces. The wavefront error must be calibrated in order to meet the demanding accuracy requirement of precision aspheric surface metrology. We propose to develop a technology that enables accurate calibration of the writing error, such that the CGH still measures an aspheric surface to an excellent accuracy of a couple of nm rms. The goal of phase 1 is to develop the software tools for calibrating the CGH writing error, and fabricate a couple of 9 inch diameter CGHs to experimentally validate the technology.

All large aspheric optical surfaces to be launched into space can benefit from accurate metrology with large CGHs. In particular, the thin shell mirrors for the X-ray telescopes, fabricated with a slumping process, can be measured with large CGHs to high accuracy, so can the mandrels for making these mirrors. The JWST primary segments, already measured with 6" CGHs, could be better tested with larger CGHs as well.

Large aspheric surfaces used in ground based telescopes, the lithographic lenses, the Extreme Ultra-Violet (EUV) lithography systems, as well as the large format aerial photography cameras will be better tested with large CGHs.

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