P.Napier, 22 January, 1998.

A meeting of the MMA antenna working group was held by phone on 17 December, 1997. Attendees included from NRAO J. Cheng, D. Emerson, J. Lugten, J. Mangum, P. Napier; from U. Az J. Bieging; from BIMA M. Fleming; from OVRO J. Lamb and D. Woody. The agenda for the meeting included:

(a) Current plan for MMA antenna work.

(b) OVRO ideas for new antenna design.

(c) Future meetings.

(a) Plans for MMA antenna work.

P. Napier presented the results of discussions within NRAO of the plan for the Design and Development Phase of the MMA. Key points of the antenna part of this plan, for the short term, include:

1/15/98 Submit project plan to the NSF. This plan has to be in detail for the development phase and in less detail for the construction phase. The plan is for a stand-alone MMA project and will probably be for an array of about 35 ten meter diameter antennas. This project will be developed into a joint project with international partners during the next two years.

2/23/98 NSF Review Panel reviews project plan in meeting at NSF with project staff.

5/98 National Science Board reviews and blesses the MMA Project.

6/1/98 MMA funds available. Increase NRAO antenna group staff from current 2 up to 5.

6/1/98 Decide on top level MMA antenna concept. (Computer model of reflector plus sketch of the mount). By this time the following major selections must be made:

Reflector

Large support ring vs small support ring

Light vs heavy panels

Mount

Yoke vs truss-alidade

Pointing, reference structure vs laser metrology

Azimuth bearing vs wheel and track

Direct drive vs speed reduction

7/1/98 Preliminary Design Review of the antenna design.

1/1/99 Critical Design Review of the antenna design (full computer model).

3/1/99 Start Preparation of the RFP package for antenna design and prototype construction.

6/1/99 Mail RFP

J. Lamb raised the question "should NRAO consider taking full responsibility for the antenna themselves, rather than contracting with a commercial company". The advantage of this approach is that it relieves the need to find a sufficiently experienced company and it is easy to incorporate novel design features that are discovered as the design progresses. P. Napier said that it was principally a question of the magnitude of the task and the need to keep to a well determined timescale. It would be very difficult for NRAO to build up a sufficiently large, experienced antenna engineering group to design and produce the antennas on the timescale needed. It is cost-effective to use a prime-contractor when such a large number of antennas has to be produced. The SMA experience is an example of the schedule problems that can arise when an observatory takes on full responsibility for an antenna project without help from a prime-contractor.

(b) New antenna design ideas from OVRO.

D. Woody and J. Lamb described their novel antenna design which is documented in the report available on the WWW at ftp://ftp.ovro.caltech.edu/pub/dpw/MMA_antenna_design.ps. This design incorporates many of the ideas discussed during the recent joint LSA/MMA antenna study (MMA Memo No 193). Chief features include a reflector with a wide support ring, cabin supported independently from the BUS, an open alidade mount with a CFRP pointing reference structure which corrects for mount deformations and direct drives for azimuth and elevation. M. Fleming raised the question of the safety of the direct drives in the event of power or other failure. The problem is that there is no simple way to provide braking which will be essential for an exposed antenna subject to wind forces. This issue needs careful consideration if direct drives are selected for the MMA.

(c) Future meetings.

It was agreed that meetings will continue on a monthly basis. Because of the important and complex decisions that must be made during the first half of 1998, every second meeting in the first half of the year will be a face-to-face meeting.