The High Sensitivity Array (HSA)
The High Sensitivity Array (HSA) comprises the VLBA, phased Very Large Array (VLA), Green Bank Telescope (GBT), and Effelsberg telescopes, and subsets thereof. All of these are equipped with instrumentation compatible with the VLBA observing capabilities described in the Station Signal Processing section. VLBI observations combining the VLBA with any one or more of the other four HSA stations can be requested in a single HSA proposal. Proposal deadlines for the HSA coincide with those for the VLBA alone, as described in Proposal Preparation, Submission, & Review. Further information on "Observing with the High Sensitivity Array" is available in a separate document.
VLA
The VLA is available as a single phased array ("Y27"; no subarrays) with two independently-tunable subband pairs, one polarization pair (RCP+LCP) in the A0/C0 basebands and the other (RCP+LCP) in the B0/D0 basebands. Setups also matching the VLBA PFB and DDC (4- and 8-channel) observing systems are available on the VLA. The VLA must be set up to match the VLBA; mixed modes are not allowed.
Bandwidths must be uniform across the entire VLBI array, and throughout the entire duration of the observation. In particular, VLA phasing and VLBI observing must be carried out at the same bandwidth. Subband bandwidths of 16 MHz and wider are available as a general capability. Bandwidths narrower than 16 MHz may work if the source is strong enough, but are expected to be of limited use, have not been tested, and are available only on a shared-risk basis.
Two adjunct documents: VLBI at the VLA and VLBI @ the VLA: Scheduling Hints, discuss the available phased-VLA capabilities, and provide instructions for their use.
GBT
Time available for VLBI on the GBT is very limited. Proposers should only include the GBT in the proposal if it is essential for the science and if it is clearly justified in the text.
The GBT is frequency agile (with some limitations) at all its bands.
The GBT's 6 cm receiver is similar to the VLBA's new system, but does differ in converting to circular polarization at ambient temperature. Tests have seen substantial polarization leakage between the RCP and LCP channels. Proposals to use this receiver will be considered only for total-intensity observations. Such proposals should request full dual-polarization modes for both observation and correlation, and careful calibration of the leakage terms should be included in the data analysis.
Proposals including the GBT in an HSA observation must include time to set up the telescope (pointing, focus, etc.) prior to the start of the observation. This can take 0.5-1 hour depending on the frequency.
Further information on use of the GBT may be found in the "GBT Observing Modes" chapter of the GBT Proposer's Guide, in the "VLBI Observing Using the GBT" chapter in the GBT Observer's Guide, and on the VLBI at the GBT webpage.
Effelsberg
The Effelsberg telescope supports both of the VLBA observing systems, and is frequency agile at 5 GHz and above.
Further information on use of Effelsberg is available at the Effelsberg HSA page.
