28 July 2014 Metrology and surface figuring of the LMT secondary mirror
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Abstract

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.

© (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
David Castro Santos, David Castro Santos, Lizeth Cabrera Cuevas, Lizeth Cabrera Cuevas, Emilio Hernández Rios, Emilio Hernández Rios, Josefina Lázaro Hernández, Josefina Lázaro Hernández, Andrea Leon-Huerta, Andrea Leon-Huerta, Maribel Lucero Alvarez, Maribel Lucero Alvarez, Carlos Tzile Torres, Carlos Tzile Torres, David M. Gale, David M. Gale, David R. Smith, David R. Smith, } "Metrology and surface figuring of the LMT secondary mirror", Proc. SPIE 9151, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, 91513R (28 July 2014); doi: 10.1117/12.2056857; https://doi.org/10.1117/12.2056857
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