1. ALMA ANTENNA KEY SPECIFICATIONS
Operational Experience of the European ALMA antennas S. Stanghellini, R. Laing, S. Rossi, W. Wild
European Southern Observatory, Karl-Schwarzschild-Str. 2, Garching b. München, Germany
SURFACE PERFORMANCE
INTRODUCTION
The European Southern Observatory (ESO) provided 25 high-precision 12 metre diameter antennas to the international ALMA project. After a design and protoyping phase, the AEM Consortium (European Industrial Engineering, Italy, MT-Mechatronics, Germany, and Thales Alenia Space, France and Italy) assembled, commissioned and delivered the 25 units from 2010 to 2013 at the ALMA Site in Chile.
Surface accuracy less than 25 μm for all antennas independent of temperature.
This poster summarizes 5+ years of accumulated operations experience of the European ALMA antennas.
ALMA ANTENNA KEY SPECIFICATIONS
10.9 μm
13.7 μm
The antenna specifications are set for efficient submillimetre imaging from a harsh high altitude site, maximizing availability and allowing remote operation.
REMOTE OPERATION
Full remote access to all antenna subsystems
Remote restart after power failure
Remote maintenance
Sub-Reflector mechanism lubrication
Azimuth and Elevation friction checks
Tiltmeter Alignment check
Antenna balancing
Key measures to ensure redundancy include
Software automatically excludes failing drive system segments
Multiple encoder reading heads
11.3 μm
13.1 μm
Fig. 1 Surface maps derived from holography. (a) primary surface of DA41 after setting at the OSF. (b) as (a), but after two years of operation at the AOS. (c) Surface map of DA41 for the full optics (primary and secondary) derived from astroholography at 230GHz. (d) As (c), but for antenna DA43.
Overall surface accuracy error: < 25 µm rss
Absolute Pointing error: < 2.0 arcsec
Offset and tracking pointing errors: < 0.6 arcsec within 2 degrees and 15 minutes
Fast switching error over 1.5 degree and 2.0 sec: < 0.6 arcsec
On the fly mapping: scan at 0.05 deg/sec with error < 1.0 arcsec
Solar observation capability
System Level MTBF (Mean Time Between Failure): > 3 years
Surface stability to be maintained over 5 years, including transport.
RELIABILITY
Key measures to promote high reliability at 5000m
Careful selection of electronic components
High altitude analysis to take into account
lower air pressure and reduced air cooling
shorter creepage distance for electrical components
Despite these measures the full reliability specification has not been met. HVAC and Antenna Control Unit improvements are ongoing.
MAINTENANCE MANUAL
The S1000D standard, common in the aeronautic industry but new in astronomy, proved to have limitations in the field.
The Atacama Large Millimeter/submillimeter Array (ALMA) is an international partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan, together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile.