Minutes of MMA Antenna Working Group, 6 January, 1998.

P.Napier 1/7/98

An ad-hoc face-to-face meeting of part of the MMA antenna working group was held on the evening of 6 January, 1998, at the URSI meeting in Boulder. Present were J.Cheng, J.Lugten, J.Mangum, P.Napier, J.Welch, D.Woody. Subjects discussed included:

(1) Antenna design and procurement plan. The current plan is still essentially the same as the plan documented in the minutes of the 17 December, 1997, meeting. D. Woody asked the question "Since we are going to have a company design the antenna, why take the time to do our own design in-house first. Why not just write the performance specification and go to bid in a few months time". P. Napier gave several reasons for wanting to have a fairly detailed in-house concept before going to bid. We need to know that the performance specifications we desire can be achieved for the available budget. We want to consider several novel design features that the companies may not be aware of. Our own design gives us a platform for comparison with the company's proposed design when we evaluate the RFPs and when we approve the vendor's design during his detailed design phase.

(2) Antenna study items. There are a number of key issues we must study and make decisions on before we have a well defined concept for an MMA 10m diameter antenna. We need this concept before the antenna Preliminary Design Review in July 98. J. Lugten reviewed some of these issues and he will distribute a list prior to our regular January 98 working group meeting.

D.Woody believes an important question that has not yet received enough study is the issue of servo design. His preliminary study suggested that adequate pointing performance, in particular the fast switching requirement, can be obtained with an antenna having a lowest resonant frequency of approx 6 Hz. This conclusion needs to be checked using an antenna model with real mass and stiffness values for the various parts of the structure. He will work on this analysis.

J. Welch raised the question of using IR or optical offset pointing to help meet the precision pointing requirement on the MMA antenna. He points out that modern optical telescopes such as Keck have absolute pointing specs comparable to the MMA but typically fail to meet them by a factor of a few. This is not a problem for them, however, because they can use offset pointing. Jack points out that it would be possible to polish a small area on the primary and secondary MMA reflectors and to locate an optical or IR CCD in the MMA secondary focal plane. A simpler implementation, which would not remove as many sources of pointing error, would be to have an optical or IR telescope looking up at the sky through a hole in the primary, as has been done before by several mm telescopes. It was agreed that, as a minimum, this latter simple configuration should be included on the first MMA antenna to provide a means of easily quantifying the pointing performance of the mount. Jack will look into the question of the number of sources available in the optical and IR for such an offset pointing system for both day and night operation.

(3) Quadrupod Scattering Studies J. Cheng described the raytrace studies that he and J. Mangum have been doing concerning the question of ground pickup due to feedleg scattering. This study is nearly complete and a report should be written soon.