When the VLA was designed, most astronomers were not overly concerned
with good sensitivity to low surface brightness features, with imaging
of fields of view wider than its primary beam, or with imaging at angular
resolutions below that provided by the D configuration. Mosaicing had
not been developed and it was believed that, in any case, such issues
were better addressed by large single dish instruments. It is now
recognized that compact arrays with total power capabilities fill a
gap between the imaging capabilities of conventional interferometer
arrays and those of large single dishes. An ultra-compact E
configuration with maximum baseline lengths of a few 
m
would provide this capability.
Given that the Arecibo telescope (for example) samples a similar
aperture how, specifically, do the capabilities of an E configuration
compare with those of such a large single dish? The single dish has
superior point-source sensitivity--the 225-m aperture that is illuminated
at Arecibo would be about 3 to 4 times more sensitive than the proposed E
configuration. The basic advantage of an ultra-compact array is its
imaging capability, e.g., the proposed E configuration could be
roughly 10 times faster than Arecibo for survey work. Hence, the
main rôle of the E configuration is to provide a fast,
low-resolution wide-field imaging capability via mosaicing. The VLA
also has access to 85% of the sky (unlike Arecibo, from which
of the sky is visible), and will provide frequency
coverage up to 50 GHz. As an interferometer, the VLA also has lower
systematic errors than a large single dish; it is less susceptible to
pointing errors, and ground pickup is uncorrelated between antennas,
as is RFI in many cases.