VLBA + Three VLA Antennas (Y3)
This document describes the particular case of three VLA antennas (Y3) being used in conjunction with the VLBA‡, with each of them correlated with the VLBA antennas and among themselves. This VLBA + Y3 mode has been partially commissioned through RSRO projects, but needs further commissioning. See the VLBA Observational Status Summary (OSS) for more details about RSRO projects.
The intention of Y3 in the context of this document is to provide VLBI with additional short spacings when combined with the VLBA. However, the use of Y3 limits the bandwidth on the VLA antennas. Relative to Y1, Y3 has 3 times the collecting area but only 1/4th the bandwidth due to recording-data-rate restrictions. Therefore, while it adds multiple baselines and enhances the uv coverage, from the sensitivity aspect Y3 is roughly equivalent to adding another half-antenna.
For proposals requesting Y3, we note that:
- Y3 is only available for A-configuration when the outer antenna baselines are useful when combined with the VLBA.
- Proposals must clearly demonstrate why single VLA antenna (Y1) is insufficient for the science proposed, because Y3 projects require significantly more resources than Y1. Note that Y3 does not appreciably increase the sensitivity for continuum observations over Y1 because of the bandwidth limits on Y3 noted above.
- If there are no restrictions on scheduling and the full array is not required for the project then Y3 will be observed in the moves from from BnA (or B) to A and from A to D configurations when the outer antennas are at their A-config locations.
- If a Y3 project must be observed during full A configuration, e.g., the project is triggered, then proposals are strongly encouraged to also include a proposal for how to use the remaining VLA antennas in a regular (non-phased) VLA mode to support the proposed VLBA+Y3 science. These should be submitted as joint VLBA/VLA proposals, with the VLBA proposal including the Y3 and the VLA proposal for the remaining 24 antennas. Because most VLA proposals for A-configuration require the full resolution of the array there will, in general, be very few other projects that can be carried out with the 24 VLA antennas not being used for the Y3 experiment, therefore proposals that do not include a request for the remaining 24 VLA antennas must have a very strong justification for not using the entire array. Note again, that if the project can be scheduled in the moves between configurations this is not required.
‡There is another Y3 concept that has not yet been tested nor commissioned and we note it here for completeness. This can also be proposed under RSRO. In this other mode, known as Y3-IP, three VLA antennas are used to enable interferometric reference pointing (IP) on the three VLA antennas when observing at high frequencies (> 15 GHz) along with the VLBA. During the observing session, when not doing IP: a) the three antennas can be phased up, summed, and recorded as a single VLBI element, giving approximately three times the sensitivity of a single VLA antenna, or b) the data from one of them can be recorded for VLBI if a wide field of view is critical for the science. Since this does not need to use the ends of the arms, in fact short baselines would be preferred, this type of Y3 does not include the restrictions detailed above.
