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.
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.
- 17 Sep 2008