Universität zu Köln

Ren, Xiaodong (2023). Holography Measurement for Crossed-Dragone Type Telescope & its Application to the Fred Young Submm Telescope. PhD thesis, Universität zu Köln.

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Abstract

Microwave Holography is an accurate and efficient method for measuring the surface shape of large reflector antennas. The method is based on the Fourier transform relationship between the antenna's far-field diffraction beam pattern and its aperture field. Measuring the antenna's far-field beam both in amplitude and phase can deduce the aperture field distribution. The phase deviations of the aperture field are directly related to the antenna's surface shape. This technique has become a well-established method for surface metrology of large radio telescopes because of its high efficiency and measurement accuracy. However, employing the traditional holography cannot identify the surface deformity in a 'two-reflector' antenna system. This thesis investigates a new multi-map holography metrology to overcome this limitation. The new method is developed to align the Fred Young Sub-millimeter telescope (FYST), a coma-corrected Crossed-Dragone antenna with two 6-m off-axis reflectors. The surfaces of the two reflectors must be aligned to be better than 10.7um. The multi-map holography identifies the surface errors between the two reflectors by taking five holographic beam measurements by placing the receiver at well-separated points in the focal plane. The parallactic shift of the surface errors allows assigning them to either one of the two mirrors. A new data processing technique is developed using an inference technique to simultaneously analyze the five beams and convert them to two surface error maps. Extensive numerical simulations have been carried out to check the feasibility, measurement accuracy, and optimum set-up of the new holographic system by modeling the systematic errors in the system, such as random instrument noise and fluctuation of performance of the instruments. These indicate that a measurement accuracy of ~2um is achievable. The critical part of the data processing technique of the 'Multi-map' holography is to develop a fast and accurate beam simulation algorithm. The conventional physical optics method is very time-consuming for analyzing the FYST antenna. A new 'two-step' Kirchhoff-Fresnel diffraction method is developed, which, compared to the conventional physical optics analysis, can reduce the computational time by four orders of magnitude without noticeable accuracy degradation. The new multi-map holography and its data processing technique are implemented to measure the reflector errors for a 0.4-m diameter Crossed-Dragone antenna in the laboratory. The experiments prove that the errors on the two reflectors can be discriminated and accurately measured with a statistic error lower than 1um. The holographic measurements and reflector corrections also indicate that the large spatial errors existing on the two reflectors also can be measured.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Ren, Xiaodong ren@ph1.uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-714446
Date:	25 October 2023
Place of Publication:	Köln
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Mathematics and Science Education > Institute of Physics Education
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Microwave holography English Radio telescope English Crossed-Dragone antenna English
Date of oral exam:	16 October 2023
Referee:	Name Academic Title Stutzki, Jürgen Professor
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71444