14.5 Averaging

14.5.1 Data compression

One may simply wish to average several channels to increase the signal to noise ratio (use tasks UV_COMPRESS and UV_AVERAGE).

14.5.2 Circular averaging

For sources with circular symmetry it may be necessary to obtain the variation of amplitude with antenna spacing, in order to compare the amplitude data with models. For this purpose, with task UV_CIRCLE one takes the mean of all the visibilities in concentric rings in the $uv$ plane. The output has the format of a $uv$ table (except that all $v$'s are zero), and may be plotted with UVALL (fig. 14.5).

Figure 14.5: Example of a circular average plot (same data as fig. 14.4)

Use this sort of averaging with caution: the phase center must accurately coincide with the source position or the amplitude of the visibility average will decrease on long spacings (use UV_FIT and UV_SHIFT). One may also do this kind of averaging in separate sectors in the $uv$ plane, to check for asymmetries, provided the $uv$ plane is well sampled (task UV_CUTS).