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Please note, we are experiencing high volume submissions; you will receive confirmations of submissions in due course. Data upload (DOI): https://researchdata.up.ac.za/ UPSpace: https://repository.up.ac.za/handle/2263/51914
Analysis of thermally-induced displacements of the HartRAO lunar laser ranger optical tube : its impact on pointing
The Hartebeesthoek Radio Astronomy Observatory (HartRAO) of South Africa is
developing a Lunar Laser Ranging (LLR) system to achieve sub-centimetre range precision to
the Moon. Key to this high precision expectation, which includes improving the overall
operational performance of its telescope, is the thermal analysis of the telescope structure. In
this study, thermal sensors were mounted on the thermally- important areas of the tube
structure to measure the tube displacements emanating from the varying ambient air
temperatures. A laser distance-measurement system was used for this purpose. Results showed
that while the optical tube undergoes structural changes with changes in temperature, the tube
position closer to the place where the spider assembly is mounted is unevenly displaced in three
directions. In particular, for the time period considered in this study, it was found that the
relative displacements on average at prisms 1, 2 and 3 in the vertical direction were 2.5540 ±
0.0007 m, 1.3750 ± 0.0008 m and 1.9780 ± 0.0007 m, respectively. The corresponding standard
deviation (SD) values of ±0.0007 m, ±0.0008 m and ±0.0007 m denotes the average deviations
that occurred in the vertical direction at the centre of prisms 1, 2 and 3, respectively. The
generally higher SD of relative displacements in the vertical direction rather than in the easting
and northing directions, suggest that the tube experienced greater variations in the vertical
direction. Furthermore, the lower arc of the tube front, was found to have increased variability, and therefore it was hypothesised to introduce more elevation pointing offsets than azimuth for
the LLR. This information constitutes an important input for guiding the efforts to determine
the extent of the correction needed to be fed into the LLR telescope pointing model to counteract
expected thermally induced pointing offsets.