Short-spacings measurements ALMA is composed of:
![]() This figure illustrates the tranfert function of an interferometer, in a 1-D uv plane. The Y axis is arbitrary. The black square are the position of the measured points in the uv plane. An interferometer cannot sample all spatial frequencies: the shortest baseline distance defines the shortest spatial frequency that is measured (vertical red line). Each measurement is actually a local average of the visibilities, weighted by the transfert function of each individual antenna (here approximated by Gaussian). ![]() Adding the measurement of a single-dish antenna of the same diameter as that of the array antennas (blue curve) provides the zero-spacing point. This is usually not enough to fill the central gap in the uv measurements. ![]() An array composed of smaller antennas (blue curves) can sample shorter baselines. But this does not provide the zero-spacing measurement. This is the CARMA array. ![]() A good solution to fully sample the uv plane is to combine the two previous setups: the short-spacings are measured by an array of smaller antennas, while the zero-spacing is measures by a single-dish antenna of the same diameter as the antennas of the main array. This is ALMA+ACA+the single-dish antennas. ![]() Another solution to fully sample the uv plane is to combine the interferometric data with the measurements obtained with a single-dish antenna whose diameter is larger than that of the array antennas: the single-dish provides in that case both the zero- and the short-spacings. This is the IRAM PdBI + 30m. |