Final Examination of Narak Choi for the Degree of Doctor of Philosophy in Optics
Thursday, March 29, 2012 10:30 AM to 12:30 PM
CREOL Room 102
IMAGE DEGRADATION DUE TO SURFACE SCATTERING IN THE PRESENCE OF ABERRATIONS
This dissertation focuses on the scattering phenomena by well-polished optical mirror surfaces. Specifically, predicting image degradation by surface scatter from rough mirror surfaces for a two-mirror telescope operating at extremely short wavelengths is performed. To evaluate image quality, surface scatter is predicted from the surface metrology data and the point spread function in the presence of both surface scatter and aberrations is calculated.
For predicting the scattering distribution, both numerical and analytic methods are considered. Among the numerous analytic methods, the small perturbation method (classical Rayleigh-Rice surface scatter theory), the Kirchhoff approximation method (classical Beckman-Kirchhoff surface scatter theory), and the generalized Harvey-Shack surface scatter theory are adopted. As a numerical method, the integral equation method (method of moments) known as a rigorous solution is discussed. Since the numerical method is computationally too intensive to obtain the scattering prediction directly for the two mirror telescope, it is used for validating the other three analytic approximate methods in special cases. In our numerical comparison work, among the three approximate methods, the generalized Harvey-Shack model shows excellent agreement to the rigorous solution so that it is used to predict surface scattering from the mirror surfaces.
Regarding image degradation due to surface scatter in the presence of aberrations, it is shown that the composite point spread function is obtained in explicit form in terms of convolutions of the geometrical point spread function and scaled bidirectional scattering distribution functions of the individual surfaces of the imaging system. The approximations and assumptions in this formulation are discussed. The result is compared to the irradiance distribution obtained using commercial non-sequential ray tracing software for the case of a two-mirror telescope operating at extreme ultra-violet wavelengths and the two results are virtually identical. Finally, the image degradation due to the scattering by the mirror surfaces and the aberration of the telescope is evaluated in terms of the fractional ensquared energy (for different wavelengths, square sizes and field angles) which is commonly used as an image quality requirement on many NASA astronomy programs.
BS: 2005, Seoul National University
MS: 2007, Seoul National University
Committee in Charge:
James E. Harvey (Chair)
M. G. “Jim” Moharam
Boris Y. Zeldovich
Approved for distribution by Dr. James Harvey, Committee Chair, on 03/14/2012
The public is welcome to attend.
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