Frequency Bands & Performance
The nominal frequency ranges for VLBA receiver systems are shown in Table 3. Actual frequency ranges are broader; consult the measurements reported by Hronek & Walker (1996) for details. Updates on frequency-dependent performance across VLBA bands are available at http://www.vlba.nrao.edu/cgi-bin/wbd_dir.pl. These actual 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).
(c) Different ranges within the 20-cm receiver.
(f) Different ranges within the 1 cm receiver. Continuum performance better at 23.8 GHz, away from water line.
(g) See Table 4 below for individual station details.
(j) Data rate 256 Mbps. (k) Data rate 32 Mbps.
(l) 8-station array; 4-hour integration.
Also shown in Table 3 are parameters characterizing the performance of a typical VLBA station for the various frequency bands. Columns [3] and [5] 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 [4].
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. Characteristic values tabulated in columns [6] and [7] are computed assuming, for most cases, the VLBA's upgraded 2-Gbps recording rate for continuum observations; a typical fringe-fit interval of 2 minutes; and a total on-source integration time of 8 hours. Exceptions, indicated in the table notes, apply to the fringe-fit intervals at the lowest and highest frequency bands, 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.
Antenna | Nominal | Typical | Typical | Typical | Baseline |
Frequency |
Zenith | Peak | Zenith | Sensitivity |
|
Range | SEFD | Gain | Tsys |
ΔS2048,30s |
|
[GHz] | [Jy] | [K Jy-1] | [K] | [mJy] | |
BR | 80 - 90 | 3500 | 0.039 | 135 | 28. |
NL | 80 - 96 | 4900 | 0.055 | 270 | 33. |
FD | 80 - 96 | 3600 | 0.034 | 120 | 28. |
LA | 80 - 90 | 3100 | 0.051 | 160 | … |
PT | 80 - 96 | 4100 | 0.024 | 100 | 28. |
KP | 80 - 96 | 4600 | 0.025 | 110 | 30. |
OV | 80 - 96 | 5800 | 0.020 | 100 | 33. |
MK | 80 - 96 | 4100 | 0.023 | 100 | 28. |
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 4 gives a snapshot of performance at 86 GHz for each antenna, as well as the RMS noise in 30 seconds (at 2 Gbps) on a baseline to LA, which is one of the most sensitive3 mm antennas.
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