Sci IPT meeting

Steve, Darrel, Antonio, Leonardo, Al, Koh-Ichiro, Richer, Nikolic, Wilson, Remy, Mel Wright

Al presented the first part of the agenda.

Discussion on ACA 7m review. This material was supplied after the telephone discussion:

Informal Report of the ACA 7-m Antenna PPDR

Tetsuo Hasegawa (Joint ALMA Office)

1. The Technical Specifications

The Technical Specs of the ACA 7-m Antennas has not been approved by the project. The PPDR panel reviewed the design presented by MELCO against the draft specifications. Based on the design review, the panel makes recommendations on the Technical Specifications.

2. The 7-m Reflector Made of Steel

MELCO presented a detailed design of the main dish based on steel structures. The choice of steel as the basic material has been driven by 1) need to balance about the EL axis and 2) the cost constraint. The former requirement is posed by the compatibility of the receiver cabin (that has been designed for the 12-m antenna and is excessively large and heavy for a 7-m dish) and the close packing specifications (collision radius smaller than 4.375 m).

To cope with the thermal expansion of steel that is an order of magnitude larger than those of low-expansion materials such as invar or CFRP, MELCO has elaborated an air ventilation system to equalize the temperature of the backup structure and the subreflector stays. MELCO has verified its thermal design calculations by an experiment with a 1/12 section model of actual size. The good agreement of the results gives credibility to the error budgeting presented in the design.

An obvious concern is the maintenance of the many fans, and a recommendation was made by the panel for further careful design to improve the maintenablity.

3. Conditions for 20 micron Surface

In the discussion for the Japanese participation in ALMA that occurred in 2003, NAOJ proposed that the 7-m antenna will have a better surface accuracy of 20 microns (rms). Before the PPDR, ALMA-J made a more detailed proposal that the surface accuracy is specified as better than 20 microns (rms) and 25 microns (rms) for nighttime and daytime, respectively, both meeting the Primary Operating Condition (POC) defined in the Technical Specs for the ALMA 12-m Antennas.

The statistics of the site monitor data show that more than 80% of the POC time occurs during nighttime, with the remaining 20% occurring exclusively in a few hours after the sunrise (the first 1 to 2 hours are generally out of POC because of too steep temperature rises). Because of the budgetary constraint, ALMA-J proposed to optimize the design to cover at least 80% of POC. This proposal was briefly discussed at the ASAC telecon a week before the PPDR, and the general response was that they were rather disappointed.

The original design from MELCO showed a surface error exceeding 20 microns (rms) for POC during daytime. In the course of the reviewing the error budget, however, the panel recognized that some error elements can be reduced greatly by allowing adjustment of the subreflector position. This may bring the total surface error below 20 microns. Based on this prospect, ALMA-J proposed that they will revert the specifications to better than 20 microns (rms) for POC in both daytime and nighttime.

4. Metrology of the Yoke

Close packing specification leaves no space for the antenna electronics (motor drive amplifiers, etc.) on the veranda, and MELCO inevitably designed to place them inside the yoke structure. Although MELCO has demonstrated the operation of the metrology system in the ACA 12-m antennas, the heat sources within the yoke complicate both the thermal behavior of the structure and the environment of the metrology system inside the 7-m antennas. MELCO has made a detailed thermal modeling and proposed a temperature correction of the metrology data. The panel recommended to simplify the thermal environment as much as possible.

5. Conclusion

It may take some time to fully appreciate the MELCO design of the 7-m antenna with a steel dish. We are not allowed to design an antenna freely from scratch --- instead, we have conflicting set of constraints such as a big receiver cabin and a yoke compatible with the 12-m antenna while keeping everything within the small collision radius. The design presented by MELCO is mature and compelling in most areas, and appears to meet the requirements within the budget.

MELCO agrees to address the recommendations from the panel as much as possible, and plans to evaluate the first 7-m antenna carefully to feedback to the manufacturing of the following ones. I think we have done as much as we can now to improve the design and minimize the risk, and in this sense the review was a success. I thank the panel members and all the others who contributed to it.

-- AlWootten - 17 Sep 2008

This topic: ALMA > WebHome > AlmaSci > AlmImcal > MonthlyMeetings > 17Sept08notes
Topic revision: 2008-09-22, AlWootten
This site is powered by FoswikiCopyright © by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding NRAO Public Wiki? Send feedback