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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.
The primary reflector of the Large Millimeter Telescope (LMT) Alfonso Serrano is presently composed of 84 surface panels arranged in three concentric rings, providing a 32.5 meter collecting area. Each panel comprises 8 precision composite subpanels having electro-formed nickel skins bonded to an aluminum honeycomb core. Differential thread adjusters beneath each subpanel allow for the manual removal of tip/tilt and piston errors, in addition to facilitating some fine tuning of the surface shape. An assembled panel provides a surface area of approximately 8-12 square meters.
Preparation of surface panels in 2012 and 2013 for Early Science observations made use of a Leica laser tracker. Measurement and adjustment of panels was carried out off the antenna, achieving a mean panel RMS surface error of 29.5μm for the 67 panels processed to date, with a spread of 23-37μm. A panel stability check consisting of surface walk-on tests and repeat metrology resulted in an increase in the mean surface error to 31.0μm. Following installation, in situ tracker measurements of 19 panels showed a final mean error of 45.3μm. Panels are adjusted by hand using an iterative process. In-house data processing uses fiducial marks scribed onto the subpanel molds and replicated during manufacture, to achieve accurate registration of the surface point cloud during data fitting. The number of iterations varies, depending mainly on the behavior of the differential adjusters. A well-behaved panel may be set within around 7 hours. In this paper we describe the iterative panel surface adjustment process used to date. We focus on metrology technique and data processing using the laser tracker, and present comparisons with trial photogrammetry measurements.
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