Ultra-Precision Non-Contact Metrology for Optical Shop and Alignment Applications

Abstract

New, unconventional designs for modern optical systems strive for superior performance at lower cost and volume. These compact optical instruments, with larger fields of view and faster f-numbers, call for more challenging prescriptions to meet their ambitious requirements. Freeform optical surfaces have the potential to enable these compact, high-performance systems, but are more challenging to characterize and hence to fabricate. This work enables the use of precision coordinate measuring machine (CMM) for a wide range of optical testing and alignment applications, including the testing and integration of freeform and aspheric optics. In this dissertation, the capabilities of this non-contact, precision metrology instrument have been stretched beyond its everyday applications. Advanced data collection and reduction techniques have been developed to determine the as-built prescription of optical surfaces, as well as the alignment with respect to the part and global coordinate systems. For measurement of low-order surface error and prescription parameters, when compared to interferometry and other optical shop techniques that rely on optical path difference or slope error, these techniques have greater dynamic range and, in some cases, precision that approaches the sensitivity of traditional methods. For example, the CMM, when equipped with a non-contact probe that utilizes a chromatic confocal principle, has a dynamic range measured in mm, a sensitivity measured in nm, yet does not require a null corrector or other custom-made components. In one case, the CMM was used to measure the surface error and as-built prescriptions of two freeform mirrors for a reflective telescope. The CMM was then utilized to align the telescope, as confirmed by an end-to-end interferometric test used to evaluate the system performance. In another example, the CMM was paired with a laser radar (LR) to align a different telescope that uses off-axis, polynomial aspheres in a three mirror anastigmat (TMA) configuration. The CMM enables fast determination of as-built prescription parameters and surface error without the types of systematic errors that are inherent in some traditional optical test setups, like the removal of power or other aberration from alignment of the test setup. The applicability of this work was further studied for large, convex aspheres using the spare secondary mirror for the Hubble Space Telescope (HST) and, via simulation, for large meter-class optics such as the Giant Magellan Telescope (GMT) secondary mirror segments. This measurement technique was further extended to other types of optical surfaces, like gratings and spatial filters. In the case of gratings, it is shown that one can derive as-built period, amplitude, and grating vector quickly and accurately from a few, fast scans.