The Large Millimeter Telescope Alfonso Serrano (LMT) currently has a primary reflector of 32.5m diameter composed of 84 panels, each having a surface area of approximately 10 square meters. Each panel is supported on four electromechanical actuators, allowing for the correction of tip-tilt, piston and twist. The actuators are designed to perform active surface compensation of gravity deformations as a function of elevation. Following the setting and installation of individual panels, an approximation for global alignment of the primary surface is carried out using a total station. An RMS error of 200 - 500μm is expected for this process. Final global alignment is conducted using holography at 12GHz for elevations corresponding to the location of geostationary sources. As an intermediate alignment option for the antenna at zenith, the use of a laser tracker has been explored. Global alignment of a large primary surface with a laser tracker presents the common problems related to the contact measurement of a large object in a non-metrology environment. Key issues are the stable location of fiducial points and the relatively slow data collection rate. Additionally the high altitude site (4600m, 15000ft) with mean temperatures around zero degrees Celsius, presents a challenge for our interferometer-equipped trackers. In this paper we present first results using a tracker located near the antenna vertex, and mechanical adjusters in place of actuators. An RMS error of around 100μm was achieved. Limiting factors included inadequate fiducials and slow mapping speed. Proposals for reduced data collection times and improved metrology robustness are presented.

Date Published: 28 July 2014
PDF: 13 pages
Proc. SPIE 9151, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, 91513Q (28 July 2014); doi: 10.1117/12.2056856

Published in SPIE Proceedings Vol. 9151:
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation
Ramón Navarro; Colin R. Cunningham; Allison A. Barto, Editor(s)