P.Napier, 7 December, 1997.
The November, 1997, meeting of the MMA antenna working group was held by phone on 26th November at 1500 MST. Attending the meeting were: J. Lamb and D. Woody from OVRO, J. Bieging from U.Az, J. Cheng, D. Emerson, M. Holdaway, J. Kingsley, J. Lugten, J. Mangum (observer), F. Owen (observer) and J. Payne from NRAO. The agenda included the following items:
(1) What is the best way for us to move into the 10-12 m diameter range?
(2) To what extent should we rely on active metrology to meet the pointing requirements?
(3) Short term work: feedleg study (Jingquan), independent encoder support (John L.)
(4) What should we do about anomalous refraction?
Discussion:
(1) P. Napier said that, given the current situation of a likely international partnership and the MAC recommendations, it does not seem sensible to do further work on the 8 m diameter design at the present time. There was general agreement on this point and discussion was concentrated on the question of how best to redirect the efforts of the working group into the 10 to 12 m regime. Three possible ways to do this that have been discussed within NRAO are:
(a) Work on an 11m design with the clear goal that it be scalable up or down by 1 m.
(b) Concentrate on a 10m and rely on the ESO 12m design to cover the 12m end of the range.
(c) We get the ESO 12m computer model and use it as a test bed for optimization studies suitable for both 10m and 12m.
Within the NRAO group there was a preference for option (c) and ESO has agreed to make their 12m model available. J. Lamb and D. Woody think that it is important to also investigate a more novel mount design in which a metrology system, used to improve mount pointing performance, is designed into the mount in an optimum way. They have in mind incorporating a CFRP reference structure which allows the true position of the elevation axis to be determined. J. Lugten commented that this sounds similar to the CFRP reference structure and linear transducers already designed into the 8m yoke mount. It was agreed that Lamb and Woody would provide some sketches of their ideas so that the group can access its ability to pursue this approach. The ability of the group to pursue more than one design will be determined by manpower availability and schedule requirements and these await clarification of the NSF budget plan.
(2) P. Napier suggested that we should think of active metrology in the most general sense, including tiltmeters, CFRP reference structures supporting metrology devices and laser/retroreflector systems. It is useful to think of incorporating active metrology at three different levels:
(A) Design the antenna to meet the pointing spec passively and include active metrology only to gain some additional margin on the spec or to improve the antenna beyond the spec.
(B) Design the passive antenna structure to fail the pointing spec by a factor of a few (2 or 3 say) in order to save some money, and rely on active metrology to recover the spec.
(C) Completely rely on active metrology to save the maximum amount of money possible, to the extent that the purely passive pointing performance will be poor.
Within the group there was agreement that a combination of (A) and (B) is the appropriate level for the MMA given the requirements for a fairly rapid development phase and the need to minimize maintenance activities on the high altitude site.
(3) J. Cheng reported that he and J. Mangum are carrying out a study to determine the optimum feedleg design (location, shape, number) to give best blockage and ground noise pickup performance. On the question of size there is general agreement within the group that any increase in feedleg size required to support the additional mass of a nutator stage for the subreflector (compared to the mass already needed for x, y and z linear motion stages) will be negligible. This means that there will be no loss of antenna G/T performance associated with the addition of a nutation stage so the decision whether or not to put nutators on the production run of antennas can wait until fabrication of the production run of nutators must start. It still is a requirement that the first antenna have a nutator for testing purposes.
(4) M. Holdaway and D. Woody are working on the anomalous refraction question and expect to issue a joint report which will clarify the situation.