Photogrammetry mapping and alignment of the LMT 50-meter primary reflector

摘要

The Large Millimeter Telescope (LMT), located in central Mexico, saw completion of the final construction phase in 2017 with the installation of the full 50-meter primary reflector, following three years of operation as a 32-meter facility. The task was accomplished by adding two more concentric rings of surface segments to the existing three inner rings. Various techniques have been used previously to measure and align the 32-meter surface, including multiple laser trackers and far-field phased holography. Whilst the former method is time-consuming, requiring a full night to obtain a single surface map, holography provides low spatial resolution and requires removal of the secondary reflector for installation of the twin-horn receiver at the primary focus. Photogrammetry has been used as an alternative measurement technique for the 32-m primary since 2015~1, and has gradually replaced our use of holography and laser trackers for this task~2 during recent years. Once the object has been targeted, photogrammetry maps may be obtained in around one hour. The technique does not require the installation of special equipment on the antenna, and has the advantage of allowing surface maps to be taken at any chosen elevation. The main drawbacks for the LMT application are environmental, since the antenna operates without an enclosure; strong winds may prevent use of the site tower crane for image taking, while the formation of condensation and frost on the reflector surface will "switch off" the reflective targets. In this paper we discuss comparative measurements taken as the first outer segments were installed, and the use of photogrammetry to carry out the alignment of the fully installed 50-meter surface. At the time of writing this activity is still in progress, however full-surface alignment to the order of just over 100 microns was achieved quite quickly, with multiple elevation maps allowing the development of a usable 50-m active surface model for compensation of gr