Physics – Optics
Scientific paper
Oct 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........17l&link_type=abstract
Thesis (PhD). UNIVERSITY OF ALABAMA IN HUNTSVILLE, Source DAI-B 60/11, p. 5590, May 2000, 196 pages.
Physics
Optics
1
Scientific paper
An optical system consisting of two Fresnel lenses is designed and proposed for use in a space-based giant air shower observatory called the Orbiting Wide-angle Light- collector (OWL). This observatory will attempt to detect more than 100 of the highest energy cosmic rays (E > 101, eV) every year. These cosmic rays are of the utmost importance in contemporary cosmology because existing theories cannot explain how they are created or how they reach the earth from their initial source. Existing hypotheses can only be verified or rejected in the presence of experimental data, and only an optical system which observes the earth's atmosphere from space is capable of providing an aperture large enough to detect a statistically significant number of events in a reasonable amount of time. This dissertation is concerned with the development and analysis of Fresnel lenses for low-resolution applications such as OWL which must search the sky for random events. Extensive computer modeling is performed using a variety of different types of optical design and analysis software in order to investigate the properties of Fresnel lenses. Of particular concern are problems associated with stray light such as facet vignetting and veiling glare. These parameters are completely characterized for various Fresnel lens systems which meet the OWL system specifications. A geometric model which does not require ray tracing is also developed and can provide information about light losses in Fresnel lenses. This model is used to analyze the effects that Fresnel lens surface shape and configuration have on facet vignetting. A basic diffractive analysis of Fresnel lenses is formulated, and it is shown that diffraction effects will not significantly alter the behavior of the OWL-scale system. Two scaled-down prototype OWL systems are designed and fabricated using a diamond turning lathe, and a consistent method for developing the tool paths for constructing complex Fresnel lenses is developed. The prototype optical systems are tested for surface figure as well as optical performance to ensure that the fabricated optics coincide with the designed and modeled optics. The optical tests include standard resolution and image quality tests as well as illumination falloff and scattering tests. The results of the research indicate that the Fresnel lens system is a suitable candidate for the OWL optical system. The feasibility of fabricating double-sided Fresnel lenses on spherical substrates is demonstrated by the prototype optics, and methods for scaling the fabrication methods to larger diameter lenses are discussed. The validity of the computer models is proven by the test data, and the geometrical model is verified by computer modeled ray tracing data. The development and verification of the computer models are the most significant accomplishments of this dissertation. These models will be relied upon to predict the behavior of large optical components without the aid of full-scale prototypes which would be prohibitively costly to manufacture. Although this research is concerned with the use of Fresnel lenses in imaging applications, the principles developed here can be applied to Fresnel lenses used in non-imaging and concentration applications as well.
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