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Land Surveying

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Browsing Land Surveying by Author "Combrinck, Ludwig."

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    Characterization and development of optical components for the Cassegrain telescope and laser beam coudé path of the lunar laser ranger of HartRAO.
    (2015) Nkosi, Nokwazi Purity.; Combrinck, Ludwig.; Akombelwa, Mulemwa.
    The Observatoire de la Côte d’Azur (OCA) donated a 1-m Cassegrain telescope to be used for the dual satellite and lunar laser ranging system currently under development at the Hartebeesthoek Radio Astronomy in South Africa. As the very first of its kind in the Southern Hemisphere, the new system will be designed and developed as a permanent lunar laser ranging system with high precision laser and electronic equipment to achieve millimetre accuracy. Limited technical details of the telescope exist so tests were conducted to determine the optical characteristics and performance of the telescope and its mirrors. The optical performance of the telescope was validated through the analysis of transmission efficiency, structural efficiency and image quality. Spectroscopic measurements were conducted to determine the transmission efficiency of the telescope by taking into account all losses in light from the reflection of mirrors, transmission of lenses and the secondary spider central obstruction along the path of the proposed coudé optical path. A system transmission of ∼90% was obtained if a coudé path with no central obstruction is used. The primary mirror and its support structure was validated using finite element analysis software (ANSYS) to model the amount of deformation the mirror will experience under gravitational and external loading. Taking into account the lightweight nature (honeycomb structure) of the mirror, its material properties and multiple support mechanism, ANSYS was used to compute the gravity deformations experienced by the mirror as the telescope tracks from the horizon to zenith. The deformations when gravity acts along the axial support were in the range of 1/6th of the wavelength, which is below the maximum limit expected for such a structure at the given weight. In order to analyse the image quality of the system, an optical analysis software (OSLO) was used. Spot diagram analysis revealed coma as the dominant primary aberration in the system. The telescope is diffraction-limited for on-axis performance and yields a Strehl ratio of 0.78 for off-axis performance.
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    Development of an integrated model and system to enable optimal efficiency of the HartRAO LLR signal path.
    (2017) Ndlovu, Sphumelele Colin.; Combrinck, Ludwig.; Akombelwa, Mulemwa.; Chetty, Naven.
    The Lunar Laser Ranger (LLR) system under development at the Hartebeesthoek Radio Astronomy Observatory (Hartford) in South Africa is being built to accurately measure the Earth-Moon distance (at 1 cm level) through the use of short laser pulses, a single photon detection system, an accurate timing system and other sophisticated components. This LLR system is unique in Africa and indeed in the entire Southern Hemisphere. The system utilizes a 1 m diameter optical telescope, which was donated to the project by the Observatoire de la Côte d’Azur of France. In this work, the author discusses the development of an integrated model that will be utilized to obtain optimal efficiency of the HartRAO-LLR system. The model is used to estimate the expected number of returned photons by considering a number of parameters which affects the laser beam pulses as they traverse the atmosphere from the LLR telescope to the Moon and back to the telescope. Factors such as the apparent Earth-Moon range, atmospheric extinction, laser beam characteristics, optical path efficiencies and others, affect the estimated (predicted by software) and actual (measured) number of returned photons for the HartRAO-LLR station. The estimated average signal return rate (which is dependent on a number of factors) of the HartRAO-LLR ranges between 0 to 12 photons per minute, which is in agreement with the available data from five globally distributed LLR stations. It also correlated with the estimated returns that were obtained using least squares parameter estimations. They were in agreement by an average difference of 0.00272. Our estimated signal returns are strongly affected by two-way atmospheric extinction (atmospheric and cirrus cloud transmissions), variations in the laser beam incident angle on the retroreflectors located on the Moon as well as the varying Earth-Moon range. A new parameter, named lunar reflectivity ranging between 0 and 1, was introduced in the link budget equation to consider the effects of Moon Phases on the returned photons. Modelling the returned number of photons and comparing these to the actual number received leads to an understanding of the effects of numerous variables on the total laser path efficiency. Total system efficiency can be improved as well, as particular atmospheric conditions will not allow LLR to be successful on certain days. For these days, the system can be utilized for other purposes such as maintenance or satellite laser ranging.