A project that will modernize the VLA electronics systems and improve all key observational parameters by a factor of 10 is under way. Details of this Expanded VLA (EVLA) Project may be found on the web, at http://www.aoc.nrao.edu/evla/ .
During the entire VLA Expansion Project we are committed to keeping the VLA observing and producing forefront science. It is expected, however, that there will be some periods when the amount of observing time is reduced, and the average number of antennas available may be fewer than for the nominal VLA. To keep people informed of the impact of the EVLA construction on VLA observations we maintain a web site at: http://www.aoc.nrao.edu/evla/archive/transition/impact.html . At the web site are short-, medium- and long-term forecasts. Users should consult these forecasts before proposing for time or observing with the VLA.
We now are returning retrofitted EVLA antennas to the VLA to be used as part of normal VLA observations. These antennas have new feed systems, wideband electronics, and fiber-optic data transmission systems; their data are ``translated'' (and reduced) considerably to be correlated against VLA antennas in the VLA correlator. Although we have put considerable effort into making it possible to observe with mixed arrays of VLA and EVLA antennas, fundamental hardware differences (e.g., mismatched bandpasses) lead to a number of complications, which often must be taken into account in constructing observing programs or in analyzing data. Although some information is provided in this document about observations using VLA+EVLA antennas, details change frequently; up-to-date information is provided at http://www.vla.nrao.edu/astro/guides/evlareturn/, and must be consulted before VLA observations are undertaken.