Question: On a tour of the VLA I learned of the 4 configurations, but I couldn’t get an answer as to why different configurations. I’ve always heard the value of VLA was the hugeness of it, in effect a 40 mile “dish” as far as angular resolution is concerned – so WHY bring the dishes close together? — Charlie
Answer: When the VLA antennas are spaced such that they are the furthest apart that they can be, the VLA is indeed able to make very high resolution measurements that can pinpoint objects in space very accurately. This ability to very precisely measure objects on the sky comes at the expense of sensitivity to the spatial structures in objects that is more extended. For example, we observe point-like objects, like stars, in distant galaxies. Those distant galaxies also contain gas and dust which is more extended. To measure these more extended structures in galaxies we move the VLA antennas closer together, which improves our sensitivity to more extended structures.