Dynamic Measurement of the ALMA Project Antenna Settings
SETIS - ALMA Project - France
The Atacama Large Millimeter Array (ALMA) is a project which consists of building an interferometric network of 66 mobile radiotelescopes located in Chile on a plateau of the Atacama desert at an altitude of 5100 m. Detailed and sustained measurements have revealed that the sky above the Atacama desert presents unique conditions of transparency and stability. ALMA will enable studying stars, galaxies and other 2 3 objects in the universe by collecting the „light“ they emit with millimetric and sub-millimetric waves. ALMA will be a unique instrument in the sense where it will enable producing detailed images, both in continuum and ray spectrum, of galaxies in formation, stars, planets, and interstellar clouds containing the chemical composites necessary for life to develop. Its approximate budget of one billion Euros is distributed between Europe (by means of the ESO), the United States (by means of NRAO) and Japan (by means of the NAOJ). These are some of ALMA‘s main research topics: -
The study of the young universe by means of observing distant galaxies - The study of star formation by means of observing molecular clouds (such as Orion for example)
- The study of planet formation
- The quest for exoplanets by astrometry
The study of the solar system based on the study of dust and the atmospheres of different planets such as Mars and Venus (the latter topic will enable having a better idea of their atmospheric dynamic or detecting the existence of water for example) Each antenna is a parabolic dish 12 m in diameter, 22 m in height, which weighs 115 tonnes and can resist a temperature varying between -20°C and +20°C.
Different Supporting Projects
The company SETIS has been entrusted with different supporting projects within the scope of the European part of the project that includes the manufacture of 25 antennae by the AEM consortium (Thalès, EIE and MT-Mechatronics). During the initial manufacture phase of antenna prototypes, the challenge was both the carbon assembly of the cabin in addition to the antenna itself, on the test site in New Mexico. The assembly project by measuring the different aspects of the antenna carbon structures consisted of measuring carbon domes for mechanical joining to the panel that serves as an antenna. No less than 2560 mechanical interfaces have to be set! SETIS‘ mission was to look for the most adapted measurement solutions and their implementation (methodology, recommendation, support...) to optimise the enormous amount of work to perform.
The manufacture of the back-up structure in Italy with drawing up of monitoring and assembly procedures on site was the second industrial phase of the project. SETIS therefore appointed a person on site in Chile to be responsible for implementing the first few antennas and training the operators.
The full range of Leica Geosystems industrial measurement instruments was implemented by SETIS during all the manufacture phases of the first antenna:
- The civil engineer receiving the antenna: size monitoring to verify its specifications compliance with a Leica Absolute Tracker AT901 at +/- 25µ, exterior, under tent, in a volume of 4500 x 4500 x 300 mm. The metallic structure during manufacture of an antenna (in Spain) and during its integration on site: size monitoring to verify its specifications compliance and deformation monitoring in Chile with a Leica Absolute Tracker AT901 at +/- 100 µ, in a provisional „container“ in a volume of 4500 x 4000 x 8000 mm
- The carbon fibre cabin during its integration in Chile: monitoring of deformation and alignment verification with a Leica Absolute Tracker AT901 at +/- 100 µ, in a provisional „container“, in a volume of 4500 x 4000 x 9000 mm
- The reflector structure (Back Up Structure) during its assembly in Chile: monitoring of deformation and alignment verification with a Leica Absolute Tracker AT901 at +/- 100 µ, in an air-conditioned building and in a volume of 12000 x 12000 x 4500 mm
- Setting the panel supports of the reflector: more than 600 “adjusters” (feet supporting the panels) were set to +/- 0.10 mm with Leica LTD640, in an air-conditioned building and a volume of 12000 x 12000 x 2000 mm.
After automation by a range of software measurements, these measurements will be taken almost autonomously by the Chilean teams of mechanics and assembly workers with no knowledge in metrology. A total of 120 reflector panels were set with Leica LTD640 at +/- 0.05 mm. The final surface area of the parabolic dish presented RMS=30 µm by monitoring 600 points.
The final surface area of the reflector was monitored at +/- 25 µm with a Leica Absolute Tracker AT901 and performed outside, with no protection and at night to avoid sun rays, in a volume of 12000 x 12000 x 2000 mm.
SETIS also verified the antenna’s kinematics, that is, its horizontal rotation movement in addition to its relative geometry. A Leica Nivel230 enabled monitoring the verticality (integrating the oscillation of the rotation axis) with a tolerance of 15Arcsec. The antenna’s geometric characteristics, that is, the orthogonality of the azimut axis (principal axis) compared to the elevation axis (horizontal axis) were measured dynamically with a laser tracker at 1.8 ArcSec.
Finally, SETIS performed deformation tests of carbon fibre structures with measurement uncertainties of approximately 10 µm over a distance of 3 m for 24 hours and then for 12 hours with measurement results at 3 µm over a distance of 4 m. Between the prototype and industrialisation, the Grenoble-based company has contributed to the evolution of measurement methods to enable a mass production of antennae.
Gaël Archambeau, ALMA project manager for SETIS, concluded “The Leica Geosystems tools have enabled dynamic measurement of the settings whilst at the same time offering the precision demanded by the client. Laser trackers are reliable under all atmospheric conditions and SETIS guarantees obtaining exact measurements, even when these approach micron precision. We have been able to rise up to this challenge thanks to a close and longlasting collaboration with the Hexagon Metrology team.