Assessment of the GBT as a short spacing provider for ngVLA Science Programs

The Next Generation Very Large Array (ngVLA) as currently conceived comprises approximately 244 18-m diameter antennas with offset Gregorian, feed-low optics. Considering mechanical clearances, these antennas may not be able to be placed closer than 40m from each other. In order to measure spatial features on the sky that are larger than what can be measured with a 40m baseline, the current ngVLA reference design calls for an array of 19 6-m diameter antennas (shortest allowed spacing: 11m), and an array of approximately 4 18-m antennas operated in total power mode to measure the “zero spacing”. This Short Baseline Array (SBA) and Total Power Array (TPA) is described in ngVLA Memos 43 & 67. Given that the ngVLA will cover the same frequency range as the Green Bank Telescope (GBT) it is natural to consider whether the GBT could be a viable alternative to the SBA+TPA combination in the current reference design. Preliminary estimates of the amount of observing time required to provide GBT data complementing ngVLA observations suggest that this could be feasible (ngVLA Memo 14) but are insufficiently detailed to constitute a realistic evaluation. For example, the ngVLA and GBT system temperatures were assumed to be equal, and the GBT observing time requirements could not be evaluated against available 3mm conditions at the site because no model of ngVLA single-dish demand as a function of wavelength existed at the time. We evaluate the feasibility of using the GBT to fulfill the short-spacing and/or total-power requirements of the ngVLA in the context of a realistic, year long observing program (EOP), considering expected instrument sensitivities and available observing conditions as a function of wavelength. With planned improvements to the GBT 3mm receiver array (Argus), the GBT could keep up with the ngVLA EOP across all frequencies.