VLA+LWA (eLWA) Observing

The VLA can be used to record individual antenna signals (voltages) to VLBI Data Interchange Format (VDIF) files. Similar to Very Long Baseline Interformetry, using the phased-VLA or individual antennas, this specifically allows for joint correlation of VLA antennas with the Long Wavelength Array (LWA) stations in New Mexico in the overlapping 4m-band range. Joint correlation is performed offline using the LWA software correlator, which is located inside the VLA control building and produces FITS-IDI compatible data output.

For shared risk observing, this mode is available for a single subband with a center frequency of 76 MHz, a bandwidth of 8 MHz, and 4-bit VDIF output. Other possible modes or center frequencies (limited by the VLA MJP-dipole response) are available through resident-shared risk observing. Use of any RSRO modes should also be brought to the attention of the LWA Director in order to verify that correlation is possible.

eLWA proposals that are granted VLA time are automatically granted time for the LWA stations, for more information on how to select the eLWA resource for your proposal, refer to the PST manual. Observations are prepared and scheduled like any other VLA observations through the OPT and the LWA stations are automatically triggered to follow the VLA pointing. Specific instructions for eLWA instrument setups are provided in the OPT manual.

The two LWA stations currently available in this mode are: LWA1 (close to the center of the VLA) and LWA-Sevilleta (on Sevilleta National Wildlife Refuge, ~80 km North-East of the VLA).

Current limitations

• VLA+LWA (eLWA) observing cannot be mixed with other observing modes in a single scheduling block/observing script. This limitation is due to the disabling of the array geometric delay model.
• Data quality is highly susceptible to the low-frequency interference environment. Known sources of interference can be the active sun, powerline arcing, or self-generated interference from AC power components or digital electronics. The environment is regularly monitored and mitigation measures are taken, which in rare cases can significantly delay or prevent execution of observations.
• Correlated data products are available through the LWA archive only and will eventually also be available through the regular NRAO archive. The PI of a proposal will be notified when correlated products are available.
• Data calibration is recommended to be performed using AIPS.