Analysis of thermally-induced displacements of the HartRAO Lunar Laser Ranger optical tube: impact on pointing

Abstract

The Hartebeesthoek Radio Astronomy Observatory (HartRAO) of South Africa is developing a Lunar Laser Ranger (LLR) system to achieve sub-centimetre range precision to the Moon. Key to this high precision expectation, which includes the overall telescope operational performance, is thermal analysis of the telescope structure. In this study we mounted thermal sensors on thermally-important areas of the tube structure and measured tube displacements due to varying ambient air temperature using a laser distance-measurement system. We found that while the optical tube undergoes structural changes with change in temperature; the tube position closer to where the spider assembly is mounted, is unevenly displaced in three directions. For the time period considered in this study, it was found that on average, the smallest to largest tube relative displacements measured plus standard deviation were in the following order per prism: LLR 1 ∆:  0.0880 ± 0.0005 m (easting), 1.2760 ± 0.0005 m (northing) and 2.5540 ± 0.0007 m (height); followed by LLR 3 ∆: 0.7380 ± 0.0005 m (easting), 1.9060 ± 0.0005 m (northing) and 1.9780 ± 0.0007 m (height); and LLR 2 ∆:  0.1660 ± 0.0004 m (easting), 1.4200 ± 0.0007 m (northing) and 1.3760 ± 0.0008 m (height). In particular, the lower arc of the tube front was found to have increased variability compared to other areas on the tube front, and it is therefore hypothesised to introduce more elevation pointing offsets than azimuth for the LLR. This information constitutes important input for guiding our efforts to determine the amount of correction needed to be fed into the LLR telescope pointing model to counteract expected thermally-induced pointing offsets