03. Antenna Evaluation at the OSF

● What is Antenna Evaluation?
The ACA antenna manufactured in Japan (for the ACA antenna, see the previous article ) is shipped from Japan to Chile. Before going to the final destination, the ALMA Array Operations Site (AOS) at 5000 m asl in the Atacama Desert, the antenna stops over at the ALMA Operations Support Facility (OSF) at 2900 m asl for assembly and evaluation. In the evaluation tests, the performance of the antenna is carefully checked and verified that it meets the performance requirements for the scientific observation.

Among various evaluation test items, we spend a lot of time measuring the two most important performance: surface accuracy (smoothness of the main reflector) and pointing accuracy (capability to keep the accurate direction to the target object). The surface accuracy is measured to an accuracy of a few micrometers (for comparison, the thickness of a plastic wrap is about 10 to 20 micrometers) by a method called “holography” using an artificial radio source. When the test results indicate any displacement of reflector panels, we conduct panel adjustment (see Figure 1) to achieve high surface accuracy as expected. We check the pointing accuracy with a small optical telescope installed in the main reflector by capturing an image of a target star whose position is identified and then measuring the difference between the actual position and the observed position. As explained above, what we are doing in the evaluation test is observing the behavior of the antenna, instead of observing the astronomical objects with the antenna.

The ALMA antenna is required to be fully operational regardless of day or night, under both high and low environmental temperature, and even under a windy condition. It is not an easy task to verify such operability by actual testing. Since the tests are conducted day and night at the OSF, we sometimes have to work all through the night (see Figure 2) or work outside for surface adjustment. I feel the antenna evaluation is a kind of physical labor.

● Staying at the OSF
For these evaluation activities, NAOJ ALMA antenna team members visit the ALMA site in Chile by rotation. During the visit over a 3-to-4 week period, we concentrate on the evaluation tests at the OSF, which is complete with dining and accommodation facilities. Although the OSF is located at an altitude lower than the Array Operations Site (OSF), it still remains “high site”. Most of the staff adjust themselves to the high altitudes and can lead a near-normal life as at sea level, but we become tired more easily and sometimes feel mental fatigue because of the closed and isolated environment of the OSF. On our days off, we try to relax ourselves by going out to a nearest village San Pedro de Atacama for a drink (because alcohol drinking is prohibited at the OSF) or by sightseeing and cycling.

One of the big problems in staying at the OSF is its dryness. The average humidity in winter is about 10 % (for comparison, about 40 to 50 % in Tokyo). If I fail to do necessary care, my hands and heels get rough and dry, and sometimes chapped. Extremely low humidity is one of the most favorable conditions for millimeter/submillimeter observation, but living in this environment is really tough.

● Antenna Acceptance Review and Handover
Now, we are in the final stage of the 12-m antenna evaluation activities that started from October 2007, and we have begun the evaluation of the newly arrived 7-m antennas. As a result of the tests, it proved that the Japanese antennas demonstrate excellent performance as expected. While we have a difficult time coping with unexpected problems and failures during the evaluation, but we feel so happy when the antenna shows us its outstanding performance that we aimed for. Furthermore, the experience of being closely involved with the antennas through the evaluation activities provides us a valuable opportunity to understand them more deeply.

The acceptance review is the final hurdle in the antenna evaluation. Based on the test results that we provided, the performance of the Japanese antenna is checked by the reviewers from Japan, Europe, and North America. They strictly check, not only the performance, but also many other items such as the safety of the antenna and documentation required for operation. When the antenna successfully passes the acceptance review, it will be handed over from NAOJ to the Joint ALMA Observatory (JAO)*. After the handover, further tests and adjustment will be conducted by the JAO staff and then transported to the AOS to be a part of the array. Further information will be provided in succeeding articles.

* Handover of the First Antenna to the Joint ALMA Observatory (Dec 19, 2008)

*The titles of authors and the names of organizations are those at the time of writing.