  Array configurations built on a hierarchy of simple elements have excellent
properties for cross-correlation imaging interferometers including a smooth
distribution of measured Fourier components, high angular resolution, low side
lobes, and compact array size. Compared to arrays with a Gaussian distribution
of antenna separations, hierarchical arrays (H-arrays) produce beams with
higher angular resolution and a tighter concentration of the total power
(encircled energy) within a smaller area around the main beam. An attractive
feature of H-arrays is their simplicity. The relationships between the Fourier
coverage and the array configuration are easy enough to understand that they
can be adjusted to achieve different design goals without the need for
numerical optimization. H-arrays will be useful for future multi-element
interferometers.
