PERFORMANCE PARAMETERS

Table 3 gives the nominal frequency ranges for the receiver/feed combinations available on all or most VLBA antennas (Thompson 1995). Passband-limiting filters are described by Thompson (1995). Measured frequency ranges are broader than nominal; consult Hronek & Walker (1996) for details and http://www.vlba.nrao.edu/cgi-bin/wbd_dir.pl for updates on the antenna performance as a function of frequency across the VLBA bands. Measured frequency ranges may be especially important for avoiding radio frequency interference (RFI), and for programs involving extragalactic lines, rotation measures (Cotton 1995b; Kemball 1999), and multi-frequency synthesis (Conway & Sault 1995; Sault & Conway 1999).

Table 3: Frequency Ranges and Typical Performance Parameters

Receivers	Nominal	Typical	Center	Typical	Baseline	Image
and	Frequency	Zenith	Frequency	Zenith	Sensitivity	Sensitivity
Feeds	Range	$SEFD$	for $SEFD$	Gain	$\Delta S^{\rm 256, 2 m}$	$\Delta I^{\rm 256, 8 h}_{\rm m}$
	[GHz]	[Jy]	[GHz]	[K Jy$^{-1}$]	[mJy]	[ $\mu{\rm Jy~beam}^{-1}$]
90 cm	0.312 - 0.342	2227	0.326	0.097	51.1 (a)	350
50 cm	0.596 - 0.626	2216	0.611	0.088	101.1 (b)	700 (b)
21 cm (c)	1.35 - 1.75	296	1.438	0.096	3.3	32
18 cm (c)	1.35 - 1.75	303	1.658	0.100	3.7	36
13 cm (d)	2.15 - 2.35	322	2.275	0.093	3.6	35
13 cm (d,e)	2.15 - 2.35	337	2.275	0.090	3.8	37
6 cm	4.6 - 5.1	312	4.999	0.130	3.5	34
4 cm	8.0 - 8.8	307	8.425	0.113	3.6	35
4 cm (e)	8.0 - 8.8	407	8.425	0.106	4.7	46
2 cm	12.0 - 15.4	550	15.369	0.104	6.2	60
1 cm (f)	21.7 - 24.1	502	22.236	0.107	5.9	57
1 cm (f)	21.7 - 24.1	441	23.799	0.107	5.1	50
7 mm	41.0 - 45.0	1436	43.174	0.078	22.2 (a,g)	151
3 mm (h)	80.0 - 90.0	4000	86.2	0.025	57. (i)	850 (j)

Notes: (a) Assumes a fringe-fit interval of 1 minute. (b) Assumes a fringe-fit interval of 1 minute and a data rate of 32 Mbps. (c) Different settings of the same 20 cm receiver. Hronek & Walker (1996) describe additional antenna-specific filters not mentioned by Thompson (1995). (d) Filters at NL, LA, and OV restrict frequencies to 2200-2400 MHz. (e) With 13/4 cm dichroic. (f) Different settings of the same 1 cm receiver. Continuum performance is better at 23.8 GHz, away from the water line. (g) Performance may be worse on some baselines due to poor subreflector or primary reflector shapes or poor atmospheric conditions (almost universal at SC). (h) ``Average'' 3 mm antennas are assumed; see Table 4 for more details. (i) Assumes a fringe-fit interval of 30 seconds and a recording rate of 512 Mbps. (j) Assumes 4 hours of integration with 7 antennas recording at a rate of 512 Mbps.

Also appearing in Table 3 are parameters characterizing the performance of a typical VLBA antenna for the various receiver/feed combinations. Columns [3] and [5] give typical VLBA system equivalent flux densities ($SEFD\/$s) at zenith and opacity-corrected gains at zenith, respectively. These were obtained from averages of right circularly polarized (RCP) and left circularly polarized (LCP) values from 10 antennas, measured at the frequencies in column [4] by VLBA operations personnel during regular pointing observations. In 2007, Germany's Max Planck Insitut für Radioastronomie funded a program to enhance the 1 cm sensitivity of the VLBA by installing modern low noise amplifiers to replace the original VLBA hardware. This program, implemented by NRAO, was completed in early 2008 and achieved its goal of reducing the zenith $SEFD\/$s by more than 30%. See Walker et al. (2007a, 2008) for more details.

The typical zenith $SEFD\/$s can be used to estimate root-mean-square (RMS) noise levels on a baseline between 2 VLBA antennas ($\Delta S$ for a single polarization; see Equation 6) and in a VLBA image ( $\Delta I_{\rm m}$ for a single polarization; see Equation 8). Characteristic values for $\Delta S^{\rm 256, 2 m}$ assuming a fringe-fit interval of $\tau_{\rm ff} =$ 2 minutes and for $\Delta I^{\rm 256, 8 h}_{\rm m}$ assuming a total integration time on source of $t_{\rm int} =$ 8 hours also appear in Table 3. The tabulated baseline sensitivities for 90 cm, 50 cm, and 7 mm assume a fringe-fit interval of 1 minute, since 2 minutes is unrealistically long. All the baseline and image sensitivities in the table, except for 50 cm and 3 mm, assume an aggregate recording bit rate equal to the typical value of 256 Mbps (see Section 6.16). This rate is commonly achieved by recording a total bandwith $\Delta\nu$ of 64 MHz (usually 32 MHz per polarization) with 2-bit (4-level) sampling (see Section 6.14). Although the original sustainable observing rate of the VLBA was 128 Mbps, advances in recording capabilities enabled the average data recording rate to reach 230 Mbps in 2007. Thus, beginning at the October 2008 proposal deadline, the default VLBA data rate for continuum observations probably will be increased from 128 Mbps to 256 Mbps. Recording at 512 Mbps is possible when required for scientific reasons and justified carefully in the observing proposal; for continuum sources, this may reduce system noise by factors of 1.4 or 2, respectively.² For 3 mm, it is assumed that twice the sustainable recording rate is used, that the fringe-fit interval is 30 seconds, and that an image is made from 4 hours of integration with 7 antennas.

Opacity-corrected zenith gains are needed for current techniques for amplitude calibration. These zenith gains vary from antenna to antenna, and are monitored by VLBA operations and communicated to users (see Section 17). The typical values appearing in Table 3 are meant to be illustrative only.

The 3 mm band is beyond the design specification for the VLBA subreflectors, and challenging for both the panel-setting accuracy of the primary reflectors and the pointing of the antennas. In addition, performance is highly dependent on weather conditions. Poor performance is the primary reason why neither HN nor SC is outfitted at 3 mm. Table 4 gives the approximate current performance at 86 GHz for each antenna, as well as the rms noise in 30 seconds (at 512 Mbps) on a baseline to LA, one of the more sensitive 3 mm antennas at present.

Table 4: Typical Performance Parameters at 86.2 GHz

Antenna	Nominal	Typical	Typical	Typical	Baseline (a)
	Frequency	Zenith	Zenith	Zenith	Sensitivity
	Range	$SEFD$	Gain	$T_{\rm sys}$	$\Delta S^{\rm 512, 30 s}$
	[GHz]	[Jy]	[K Jy$^{-1}$]	[K]	[mJy]
BR	80.0 - 90.0	3500	0.039	135	55.
NL	80.0 - 96.0	4900	0.055	270	65.
FD	80.0 - 96.0	3600	0.034	120	55.
LA	80.0 - 90.0	3100	0.051	160	$\cdots$
PT	80.0 - 96.0	4100	0.024	100	55.
KP	80.0 - 96.0	4600	0.025	110	60.
OV	80.0 - 96.0	5800	0.020	100	65.
MK	80.0 - 96.0	4100	0.023	100	55.

Note: (a) Baseline to LA is assumed.