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Woodside, Robert M. Metrology of Reflective Optical Systems. (Under the direction of Dr. Thomas A. Dow)
A functioning reflective optical system requires designing, fabricating, and assembling complicated optical surfaces. However, once these surfaces have been fabricated, an accurate metrology process must be available to ensure the fabricated optical and fiducial surfaces conform to the designed surfaces. The goal of this research is to apply metrology techniques to reflective optical systems and their components. A two mirror Ritchey-Chretién telescope was designed and fabricated as a vehicle to test these metrology techniques. This system involves two rotationally symmetric conic mirrors. A conic mirror is much more difficult to measure than a simple flat or spherical mirror because of the added variable of a second focusing location. Measuring the Ritchey-Chretién telescope components is intended to provide a knowledge base to build metrology techniques for more complicated systems such as a three mirror anastigmatic optical system. Techniques must be available to measure the individual optical surfaces, the fiducial surfaces used for assembly and the complete system.
The available techniques to measure an optical system include interferometry, profilometry, optical targets and coordinate measuring machines. The Zygo GPI Fizeau interferometer was used in a dual-pass setup to measure optical surface form error and the assembled optical system performance. The GPI can produce three dimensional form error maps and also measures wavefront error and modulation transfer function (MTF). The Zygo NewView White Light Interferometer was used to measure the surface finish of an optical surface. Profilometry with the Taylor-Hobson Talysurf Profilometer and a unique rotational profilometer were used to measure the form error of the optical surface as well as the fiducial surface errors. Profilmoetry was also applied to the primary optical surface to verify measurements made on the GPI and as an alternative to measure the secondary mirror. Measuring the flatness of the fiducial surfaces required profilometry because the fiducial surface diameters are larger than the aperture of the GPI.
Optical targets were used to measure the MTF of the assembled optical system and to measure the image producing ability of the system. Coordinate measuring machines were necessary to measure the height of the fiducial surfaces used for assembly. This measurement is necessary to determine the location of the mirror apexes upon assembly. The metrology techniques were shown to be robust for any rotationally symmetric optical system while simultaneously uncovering the advantages and shortcomings inherent in the individual metrology techniques. The process also uncovered flaws in the component and system design that reduced the available metrology options. The optical component measurement showed that the optical mirrors possessed a surface finish that is expected for the material and fabrication process. The secondary mirror showed form error as expected near λ/4 while the primary mirror had a large form error of over 3λ. The system measurement showed 1.5 μm of wavefront error at its best focus point. The wavefront aberrations present in the system caused spot sizes double the magnitude predicted by Code V and MTF values that are approximately 10-20% of the theoretical values.