Browse
Recent Submissions
Item Analysis of the accuracy of GNSS inferred precipitable water vapour against that from a 210 GHz WVR at the H.E.S.S. site(Oxford University Press, 2025) Frans, Lott; Backes, Michael; Falcke, Heino; Venturi, TizianaThe High Energy Stereoscopic System (H.E.S.S.) site and the Gamsberg Mountain have been identified as potential sites for the Africa Millimetre Telescope (AMT). The AMT is poised to observe at millimetre and possibly at submillimetre wavelengths. At these wavelengths, precipitable water vapour (PWV) in the atmosphere is the main source of opacity during observations and therefore needs to be accurately assessed at the potential sites for the AMT. In order to investigate the PWV conditions for the AMT, identical Global Navigation Satellite System (GNSS) stations were installed and used to assess the PWV at the two potential sites. In this study, the accuracy of those PWV measurements by the GNSS stations was assessed by comparing the H.E.S.S. installed GNSS station PWV measurements to that from a 210 GHz Water Vapour Radiometer (WVR) also installed at the H.E.S.S. site. A correlation of 98 per cent and an offset of 0.34 mm was found between the GNSS station and the 210 GHz WVR PWV data when on-site pressure and the Nevada Geodetic Laboratory (NGL) weighted mean temperature ( ) were used to calculate the GNSS station PWV data. In comparison, the offset reduces to 0.15 mm when on-site derived and pressure were used to calculate the GNSS station PWV. The results show that the GNSS station with on-site meteorological data can be used with high accuracy to reliably determine the PWV conditions at the H.E.S.S. siteItem A comparative analysis of GNSS-inferred precipitable water vapour at the potential sites for the Africa Millimetre Telescope(Monthly Notices of the Royal Astronomical Society, 2025) Frans, Lott; Backes, Michael; Falcke, Heino; Venturi, TizianaThe Event Horizon Telescope (EHT) is a network of antennas across the globe currently used to image supermassive black holes (SMBHs) at a frequency of 230 GHz. Since the release of the image of M87 in 2019 and, subsequently, that of Sgr A in 2022 by the EHT collaboration, the focus has shifted to dynamically imaging SMBHs. This has led to a search for potential sites to extend and fill in the gaps within the EHT network. The Gamsberg Mountain and the High-Energy Stereoscopic System (H.E.S.S.) site are both located within the Khomas highlands and have been identified as potential sites for the Africa Millimetre Telescope (AMT). Precipitable water vapour (PWV) in the atmosphere is the main source of opacity and noise from atmospheric emissions when observing at millimetre to sub-millimetre wavelengths. This study aims to establish the PWV content and the atmospheric transmission at 86, 230, and 345 GHz at the AMT potential sites using Global Navigation Satellite System derived PWV data. Results show both sites have potential for observations at 86 and 230 GHz, with 345 GHz possible at the Gamsberg Mountain during winter. The overall median PWV of 14.27 and 9.25 mm was calculated at the H.E.S.S. site and the Gamsberg Mountain, respectively. The EHT window had PWV medians of 16.62 and 11.20 mm at the H.E.S.S. site and Gamsberg Mountain, respectively. Among the two sites, the Gamsberg Mountain had the lowest PWV conditions, therefore making it the most suitable site for the AMTItem An efficient method for the synthesis of dihydropyridine by hantzsch reaction with Fe/SiO2 nano heterogeneous catalysts(2020) Rahman, Ateeq; Nehemia, P.N.; Nyambe, Moola M.An efficient method for the synthesis of dihydropyridines (DHPs) by Hantzsch reaction with Fe/SiO2 heterogeneous catalysts was developed. The Fe/SiO2 catalysts was prepared by impregnation method. The catalysts were characterized by IR and SEM instruments. The SEM results indicated that Fe/SiO2 nano spheres were formed. The reaction procedure involved reaction of aldehyde, ethyl acetoacetate (EAA), ammonium acetate (NH4OAc) and ethanol under reflux. The study was focused on optimizing reactions conditions: Standardization of catalyst, substrate of study and solvent study. In order to iden-tify the best active catalysts, five different ratios of catalyst were synthesized and evaluated for the ti-tle reaction under similar conditions. To standardize the active catalysts, different temperature condi-tions (i.e. room temperature, 60 ºC and 80 ºC) as well as catalysts amounts were evaluated. Under these established conditions, 2.5% Fe/SiO2 was the best active catalysts that resulted. Benzaldehyde and p-anisaldehyde were used to study the effect of having various substrates on the conversion and reaction time, especially the substituted aldehydes. The best results were obtained by reacting p-anisaldehyde with EAA, NH4OAc, and ethanol at 60 ºC with 0.3 grams of 2.5% Fe/SiO2 heterogeneous catalysts. Thin Layer Chromatography (TLC) monitoring of the reaction mixture showed no selectivity at high temperatures (80 ºC) with 15% Fe/SiO2. Standardization of solvent study was executed with two solvents, ethanol and acetonitrile. The product dihydropyridines were analyzed using gas chroma-tography-mass spectrometry (GC-MS). The melting points of the products were compared with authen-tic samples reported in the literature. Hence, the Fe/SiO2 catalysts is eco-friendly and economically de-veloped for the title reaction.Item Photophysical, electrochemical, and DFT studies of the novel azacrownbridged dinuclear ruthenium dye sensitizers for solar cells(University of Namibia, 2019) Uahengo, Veikko; Cai, Ping; Naimhwaka, Johannes; Rahman, Ateeq; Daniel, Likius S.; Bhakhoa, Hanusha; Rhyman, Lydia; Ramasani, PonnaduraiA dinuclear ruthenium bipyridyl complex with 1,10-diaza-18-crown-6 bridging ligand was synthesized and characterized. Its photophysical and electrochemical properties were also studied. DFT computations were performed to complement the experimental investigations. Analysis of the results indicates that the path in which charges move from one metal center to the other is significant for effective electronic coupling. However, electron transfer between the two ruthenium centers is hindered by the azacrown bridging ligand, compared to the smooth electronic transfer reported for a related dye involving an azo bridging ligand.