BIMA MEMO No. 31. B CONFIGURATION W.J.Welch, Nov-93 This memo describes a B array which, when combined with the present C array, improves the resolution, by about a factor of three, to 5". Unlike earlier B arrays, this one has only a small range of shortest to longest spacings and gives reasonable sampling of that range for a seven hour track. The present C array gives a well sampled resolution of 10" - 12" for a seven hour track. For all declinations below about 45 degrees, foreshortening provides baselines of the minimum antenna diameter, 6m. In the future, when we have the use of the rolling bases, we can add the short separation of 7m to the C array to give the shortest spacing at all declinations. The B array satisfies the following constraints. The track is only seven hours long. This keeps the antenna elevations high, above 30 degrees for declinations above about 5 degrees. The high elevations are important for millimeter observations because of the substantial atmospheric extinction. For lower declinations, lower elevations must be accepted. The other constraint that we apply is that no observations are done below 20 degrees elevation (three air masses) for any declination. The other advantage of the seven hour track is that, including the astronomical passband calibration, one total source observation takes just about eight hours. Altogether, three sources can be observed in one day. One final constraint is that this B array overlap the C array for a small range. This allows intercomparison of calibrations of the two arrays. The importance of operating at high elevation angles is shown in Figure 1. The SSB system temperature is plotted as a function of elevation angle for a receiver temperature of 50K(DSB) for a range of atmospheric optical depth. The lower three optical depths are for 4, 8, and 12 mmH|2|O at 90 GHz; the upper three optical depths are for 4, 8, and 12 mmH|2|O at 230 GHz. Opacity at CO(1-0) is about the same as it is for the CO(2-1) line at 230 GHz. Elevation angles of 30, 20 and 15 degrees correspond to two, three, and four airmasses respectively. 50K is the lowest temperature of our present SIS mixer, but we expect lower temperatures at three millimeters and values of about 50K at one millimeter wavelength in the near future. It is clear that observations beyond two or three airmasses do not produce useful data, even in the better weather conditions. The stations of the B array are at 100W, 0, 180E, 100N, 260N, and 320N. The array has a larger extent North-South relative to East-West. As a result, it has a nearly round beam at the lower declinations and is slightly eliptical at the high declinations. The uv tracks are shown in Figure 2 for declinations of 60, 20, -5, and -30 degrees. The corresponding beams are shown in Figure 3. The half-power widths for uniform weighting are shown in Table 1. Both the uv tracks and the beams are made with the combination of both the C and B arrays. _TABLE 1_ _\del_ _Beam_ 60 5.1 x 3.4 20 5.7 x 3.4 -5 6.0 x 4.7 -30 8.2 x 5.0