VLA Test/Observation Coordination Meeting B.G. Clark October 20, 2005 1. EVLA P. Napier raised a question about feeds. The antenna mechanics are having problems making the feeds fit on the feed circle. There is nominally room for them, but due to buildup of tolerances, forcing the feeds to fit on the circle is painful. The mechanical engineers are requesting to move the C, X and L band feeds outward by one inch (the L band feed is already two inches outside the feed circle; this would be an additional inch). The consequences were examined. The loss of gain for this small movement is only of order one percent, and is not that serious. The power removed from the main beam will go into sidelobes, making the sidelobe structure of the beam somewhat asymmetric (though a shaped system does not display a simple coma lobe that a simpler geometry would have). One noticeable effect is that the shaped horn feeds (L band and C band, but not X band) display a fairly large change of effective phase center with frequency within the band. If the feed is positioned off the feed circle, the quadratic phase error induced by this interacts with the change in illumination induced by being off the feed circle to produce a change in pointing as a function of frequency. Walter Brisken has run a ray-trace program that predicts a 2.7" additional change of pointing across C band due to being off the feed circle (he considers ray trace unreliable for L band because of diffraction effects, and can't estimate what this would do to L band). The consequences for deep field imaging (according to F. Owen) probably include a very substantial increase of computing required to take account of the effect. (This is also true for any high dynamic range image of a large object.) In general discussion, the feeling appeared to be that making all antennas as much as possible the same is more important than the mere deleterious effects of moving the feeds. Vivek Dhawan pointed out that the real center of the feed circle is set by where the subreflector rotation axis is pointing, and that seems to be uncertain by an amount of order 1cm, even with best holographic estimates. K. Sowinski has had a look at a baseline run with the EVLA antennas. There are anomalies, including a strong phase versus elevation effect, probably due to the different way the two systems handle refraction. This effect is several times the corrections that would be applied by the round-trip phase measuring instrument. The latter seems to show an elevation dependence, presumably due to twisting the fibers as they go through the elevation bearing.