C.L. Carilli , M.A. Holdaway 
May 22, 1997
Keywords: phase calibration, fast switching, high resolution imaging, long baselines
We present a series of tests of the Fast Switching (FS) phase calibration technique using the Very Large Array (VLA) at mm wavelengths on baselines out to 33 km. These tests demonstrate that FS phase calibration with cycle times ~100 sec can result in diffraction limited images of faint sources at 7mm in the largest configurations of the VLA under good weather conditions. There are times however when shorter cycle times may be required. We present examples using both `control sources' (ie. celestial calibrators), and a faint source of astronomical interest: the M2 supergiant star Betelgeuse (alpha Orionis). A diffraction limited resolution image (40 mas resolution) of the surface of Betelgeuse was obtained showing a resolved radio photosphere with mean T_B = 3500 K and diameter = 80 mas, consistent with theoretical models of this star.
We also present the tropospheric root phase structure function on baselines ranging from 200m to 20000m. This function shows the three regimes predicted by Kolmogorov turbulence theory: On short baselines (b <= 1.2 km) the measured power-law index is n = 0.85 +/- 0.03, while the predicted value is 0.83 (thick screen). On intermediate baselines (1.2 <= b <= 6 km) the measured index is 0.41 +/- 0.03 and the predicted value is 0.33 (thin screen). On long baselines (b >= 6 km) the measured index is 0.1 +/- 0.2 and the predicted value is zero (outer scale). The implication is that the vertical extent of the turbulent boundary layer is about 1 km, and that the outer scale of the turbulence is 6 km, although the long baseline data suggests that the outer scale may be anisotropic.
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