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.

The Large Millimeter Telescope Alfonso Serrano (LMT) is a 50-meter (currently 32m) diameter single-dish telescope optimized for astronomical observations at millimeter wavelengths in the range 0.85 mm < λ < 4 mm. During initial operation, the LMT makes use of the central 1.7 meters of a 2.5m hyperbolic secondary reflector constructed of cast and machined aluminum. Following the first light campaign in 2011, a program of iterative surface sanding was carried out to reduce the surface error of the central area to a level compatible with that presently achieved for the primary reflector. Metrology during the sanding process was conducted using a Leica laser tracker. A total of 22 sanding iterations were interspersed with tracker measurements at differing spatial resolutions, allowing the RMS surface error to be reduced from 63 to 35 microns. Maps for the final iterations were repeated for distinct scan patterns to check for systematic variance. Since the work was carried out in early 2013, repeat measurements of the dismounted secondary have confirmed the stability of this reflector.

In this paper we present details of the surface improvement program with emphasis on the metrology techniques used throughout the process. We discuss issues such as data sampling, measurement geometry, and mirror orientation. We also consider the steps taken to ensure tight control of the sanding task itself, since this process was carried out entirely by hand. Finally we present some comparative metrology results obtained using our laser tracker and photogrammetry equipment.