Characterization and development of optical components for the Cassegrain telescope and laser beam coudé path of the lunar laser ranger of HartRAO.
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.