22 July 2016 Mapping the Large Millimeter Telescope primary reflector using photogrammetry: a first comparison with 12 GHz holography
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Abstract
The Large Millimeter Telescope (LMT) makes extensive use of 12 GHz holography during maintenance periods to finetune the alignment of primary reflector segments to the best-fit design parabola. Tracker measurements have also been used for this task, however the technique is severely limited by environmental noise and large data collection times, on the order of many hours for a single map. In 2015 we started photogrammetry trials as a complimentary measurement technique. Photogrammetry can offer reduced mapping times compared with laser trackers, and like holography, allows maps to be made at arbitrary elevation angles. Depending on the placement of reflecting targets, the technique can also provide higher spatial resolution than currently achieved using our holography system.

Accurate photogrammetry requires a robust strategy for the incorporation of multiple camera stations, a task complicated by the size of the antenna, obstructions of the surface by the sub-reflector and tetrapod legs, and the practicability of using the site tower crane as a moving camera platform. Image scaling is also a major consideration, since photogrammetry lacks any inherent distance reference. Therefore appropriate scale bars must be fabricated and located within the camera field of view. Additional considerations relate to the size and placement of reflective targets, and the optimization of camera settings. In this paper we present some initial comparisons of laser tracker, holography and photogrammetry measurements taken in 2015, showing clearly the status of alignment for distinct zones of the currently operating 32.5 m primary collecting area.
© (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
David M. Gale, Andrea Leon-Huerta, Lizeth Cabrera Cuevas, David Castro Santos, Emilio Hernández Ríos, Maribel Lucero Álvarez, Esteban Tecuapetla Sosa, Carlos Tzile Torres, David Sánchez-Argüelles, Gopal Narayanan, F. Peter Schloerb, Grant W. Wilson, David R. Smith, "Mapping the Large Millimeter Telescope primary reflector using photogrammetry: a first comparison with 12 GHz holography", Proc. SPIE 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, 99124F (22 July 2016); doi: 10.1117/12.2233843; https://doi.org/10.1117/12.2233843
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