Photogrammetry mapping and alignment of the LMT 50-meter primary reflector
Photogrammetry mapping and alignment of the LMT 50-meter primary reflector
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LMT 50 米主反射镜的摄影测量测绘和对准
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发表时间：
2018
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通讯作者：
Emilio H. Rios
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作者：
D. Gale;Peter F. Schloerb;Andrea L. Huerta;Maribel L. Alvarez;Lizeth C. Cuevas;Esteban T. Sosa;David C. Santos;Carlos T. Torres;Emilio H. Rios
位于墨西哥中部的大型毫米波望远镜（LMT）在作为32米设施运行三年后，于2017年完成了最后的建设阶段，安装了完整的50米主反射器。该任务是通过在现有的三个内环上再增加两个表面段的同心环来完成的。先前已经使用各种技术来测量和对准32米的表面，包括多个激光跟踪器和远场相位全息术。虽然前一种方法是耗时的，需要一整夜来获得一个单一的表面地图，全息提供了低的空间分辨率，并需要删除副反射器的安装在主焦点的双喇叭接收器。自2015年以来，摄影测量已被用作32米小学的替代测量技术1，近年来已逐渐取代我们在这项任务中使用的全息摄影和激光跟踪仪2。一旦确定目标，大约一小时内就可以获得摄影测量图。该技术不需要在天线上安装特殊设备，并且具有允许在任何选定的海拔高度拍摄表面地图的优点。LMT应用的主要缺点是环境，因为天线在没有外壳的情况下工作;强风可能会阻止使用现场塔起重机进行图像拍摄，而反射器表面上的冷凝和霜冻的形成将“关闭”反射目标。在本文中，我们讨论了第一个外部段安装时采取的比较测量，以及使用摄影测量来进行完全安装的50米表面的对准。在撰写本报告时，这项工作仍在进行中，但很快就实现了100微米以上的全表面对准，多个高程图允许开发一个可用的50米活动表面模型，以补偿重力扭曲。
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 20151, and has gradually replaced our use of holography and laser trackers for this task2 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 gravitational distortions.