The enhanced VLA should allow measurements of cluster masses using gravitational lenses. Recent optical imaging of the lensing field of rich clusters suggests that rich clusters have deeper central potential wells and a more extended large scale gravitational potential than was previously thought. These results are limited by uncertainty in the distances of the objects being lensed and by knowing how much of the extension of the objects is produced by the lens and how much is intrinsic. The results suggest a new picture of clusters in which magnetic fields and relativistic particles could be important to their pressure balance.
Radio astronomy has contributed little to such work so far because the
VLA currently lacks the sensitivity to image enough objects in each
field. We need more sensitive imaging in the VLA
A configuration over wide fields at 
20cm of sources in the 10
to 100 microJy range. The improved bandwidth and receiver performance
of the enhanced VLA, along with the narrower channels and reduced
systematics, should enable the required sensitive wide-field imaging.