Towards development of a thermal monitoring system and analysis of meteorological parameters for the HartRAO lunar laser ranging telescope

Abstract

The Hartebeesthoek Radio Astronomy Observatory (HartRAO) of South Africa is currently developing a Lunar Laser Ranger (LLR) system based on a 1-metre aperture telescope in collaboration with the National Aeronautics and Space Administration (NASA) and the Observatoire de la Côte d'Azur (OCA). This LLR will be an addition to a small number of operating LLR stations globally and it is expected to achieve sub-centimetre range precision to the Moon. Key to this achievement, requires thermal analysis of the composite telescope structure, based on thermal properties of the telescope component materials and their interaction with the environment through conventional heat transfer mechanisms. This analysis includes a thermal monitoring system that will feed temperature measurements to a model that will assist the steering and pointing software of the telescope in order to minimize tracking errors. In particular, no study has been reported previously on the thermal behaviour and related structural changes coupled with displacements of the HartRAO LLR composite structure with respect to ambient air temperature at the observatory site. Furthermore, a prototype pointing and steering software package developed for the HartRAO LLR, has so far only been tested on a 125 mm dual refractor testbed telescope (under room temperature conditions) and achieved root mean squared error values at the 0.5 arcsecond level. The extent of variation of the achieved error values is currently not known, particularly when the pointing model will be tested on the actual LLR telescope, which will be exposed to the varying thermal environment during operation. Therefore, in this study the thermal behaviour and related distortion dynamics of the HartRAO LLR telescope composite structure were modelled for possible adverse impact on pointing. Key findings of this research study were that the thermal response time varies per LLR telescope material component, primarily due to their respective thermal properties. The spider assembly and outer tube surface had the largest range of thermal variations, and thus were identified as the main areas on the telescope where most thermal variations can be expected. However, the primary mirror surface including its mount as well as the fork assembly had the lowest range of thermal variations. The total deformations of the tube assembly were found to be in the range 2.9 μm to 40.7 μm from night (00h00) until approximately midday (11h30). The primary mirror had virtually zero localised deformations due to its resistance against temperature change. The LLR thermal dynamic model was proposed and several test results of the proposed model were presented which covered the placement of RTD sensors on thermally-important areas of the tube structure; measurement and interpolation of the optical tube temperatures; and tube displacements due to assumed thermal deformations were reported using a laser distance-measurement system. In particular, the smallest variations in relative displacements of the tube were found to be 0.418 mm (east), 0.512 mm (north) and 0.670 mm (height) whereas, the largest variations were 0.523 mm (east), 0.691 mm (north) and 0.751 mm (height) during the time period considered. This period was characterized by ambient temperatures that varied between 11.20 °C and 29.90 °C and corresponding tube temperatures that varied between 13.75 °C and 33.84 °C. This information constitutes an important input for guiding the efforts to determine the amount of correction needed to be fed into the LLR telescope pointing model to counteract expected thermally-induced pointing offsets. Overall these results are a step towards the development of a real-time thermal monitoring system for the HartRAO LLR telescope, which is imperative in maximizing the pointing accuracy of the telescope, thereby increasing the chance of being on-target with the retroreflectors located on the lunar surface. Efforts to maximize pointing accuracy for the HartRAO LLR would support the global effort of high-accuracy laser ranging, which currently provides millimetre precision. Lastly, these findings have significant implications in exploring strategies and options for developing thermal dynamic models and monitoring systems for current and future LLR optical telescopes.