Frequency Bands & Performance

The nominal frequency ranges for VLBA receiver systems are shown in Table 5.1. Actual frequency ranges are broader; consult the measurements reported by Hronek & Walker (1996) for details. The actual frequency ranges may be especially important for avoiding radio frequency interference (RFI), and for programs involving extragalactic spectral lines, rotation measures (Cotton 1995b; Kemball 1999), and multi-frequency synthesis (Conway & Sault 1995; Sault & Conway 1999). Information on the RFI environment at each VLBA station is available on the VLBA RFI webpage. 

Also shown in Table 5.1 are parameters characterizing the performance of a typical VLBA station for the various frequency bands. Columns [4] and [6] give typical VLBA system-equivalent-flux-density (SEFD) values at zenith and opacity-corrected peak gains, respectively. These are means over measurements in both polarization at all ten antennas, at the frequencies in column [5].

The typical zenith SEFD can be combined with the aggregate recorded data rate and appropriate integration times to estimate the root-mean-square (RMS) noise level on a single VLBA baseline, and in a VLBA image. The characteristic baseline sensitivity values tabulated in column [7] are computed for a single data channel assuming, for most cases, the VLBA's upgraded 4-Gbps recording rate (128 MHz of bandwidth, or a data rate of 512 Mbps, per data channel) for continuum observations and a typical integration interval of 1 minute. The characteristic image sensitivity values tabulated in column [8] are computed assuming, for most cases, the 4-Gbps recording rate for continuum observations and a total on-source integration time of 8 hours. Exceptions, indicated in the table notes, apply to the integration intervals at the highest frequency band (3-mm), where shorter intervals are often required; for the recording rate limits imposed by the available RF bandwidth at the lowest frequency bands; and for most parameters at the extreme 3-mm band. Performance may be worse than the tabulated estimates on some baselines due to poor primary or subreflector surfaces or poor atmospheric conditions.

The 3 mm band extends beyond the design specification for the VLBA antenna, and is challenging for the panel-setting accuracy of the primary reflectors, the figure of the subreflectors, and the pointing of the antennas. In addition, performance in this band is highly dependent on weather conditions. Table 5.2 gives a snapshot of performance at 86 GHz for each antenna, as well as the RMS noise in 30 seconds (at 512 Mbps, or 128 MHz bandwidth) on a baseline to LA, which is one of the most sensitive antennas with a 3 mm receiver.

Note that the 3 mm SEFD values were determined during reasonably favorable weather conditions.  Because real-world conditions are often less than ideal, noise levels in actual observations may be higher than estimated.  This is especially true for Global Millimeter VLBI Array (GMVA) and other fixed-date observations that cannot be rescheduled due to poor weather.  When planning for GMVA and other fixed-date observations, it is prudent to expect the actual SEFD values will be approximately three times higher than the values presented in the Table 5.2.

† Special Notes on the 13cm Receiver

Recent investigations by NRAO staff indicate that new sources of RFI in the 2.2 to 2.4 GHz range are causing compression in the 13cm receiver. The resulting uncertainty in flux density at 13cm, even with good calibration, is estimated to be ~50%. Users who require good flux density calibration for their science should avoid the 13cm receiver. For more details, see the VLBA Scientific Memo 41.

‡ Special Notes on the 13/4-cm Dichroic System

The 13/4-cm (S/X) dichroic system is intended for astrometric and geodetic projects which require good calibration of ionospheric effects on the phases. Due to the compression discussed above, the 13/4-cm dichroic system should not be used to obtain spectral index information. Furthermore, polarimetry using the 4cm receiver while the 13/4-cm dichroic system is in use is not well tested.