Table 5 shows the VLA sensitivities expected for natural weighting of the visibility data. The values listed are the expected rms fluctuations due to thermal noise on an image, calculated using the standard formulae with the system temperatures and efficiencies listed. A maximum number of 25 antennas is used in these calculations (except for U band); at any given time, it is assumed that 3 of the possible 28 antennas are missing, one each for EVLA mechanical retrofitting, EVLA electronics outfitting, and EVLA commissioning. Hardware limitations prevent us from inputting more than 22 EVLA antennas to the old VLA correlator, and no more than 25 antennas will be available by the end of 2009. The tabulated sensitivity values are realized in practice except at low frequencies and in smaller configurations where the sensitivities are limited by confusing sidelobes from objects outside the image. The rms limit due to confusing sources for the VLA in D configuration is estimated in Table 5. Another case where the thermal rms noise will not be achieved is in imaging very bright objects where the residual image noise is due to baseline dependent errors (`closure errors' - see 3.11).
| Frequency | Band Name | System | Antenna | Number | RMS (10 min) | |
| (GHz) | Approximate | Letter | Temperature |
Efficiency |
Antennas | Sensitivity |
| Wavelength | Code | (K) | (%) | (VLA+EVLA) | (mJy) | |
| 0.073 - 0.0745 | 400 cm | 4 | 1000-10000 | 15 | 5+20 | 160 |
| 0.3 - 0.34 | 90 cm | P | 150-180 | 40 | |
|
| 1.24 - 1.70 | 20 cm | L | 35 | 55 | 5+20 | 0.061 |
| 4.5 - 5.0 | 6 cm | C | 45 | 69 | 5+20 | 0.058 |
| 8.1 - 8.8 | 3.6 cm | X | 35 | 63 | 5+20 | 0.049 |
| 14.6 - 15.3 | 2 cm | U | 120 | 58 | 5+0 | 1.0 |
| 22.0 - 24.0 | 1.3 cm | K | 50 - 80 | 40 | 5+20 | 0.11 |
| 40.0 - 50.0 | 0.7 cm | Q | 80 | 35 | 5+20 | 0.27 |
| Frequency | Wavelength | RMS Point-Source | Beam-averaged | Antenna | Peak/Total | RMS |
| Sensitivity | Brightness | Primary | Confusing | Confusion | ||
| (12 hours) | Sensitivity |
Beam Size | Source | Level | ||
| (D-config) | (FWHP) | in Beam | (D-config) | |||
| (GHz) | (mJy) | (mKelvins) |
|
(Jy) | ( |
|
| 0.073 - 0.0745 | 400 cm | 17 |
2000 | 700 |
20/350 | lots |
| 0.3 - 0.34 | 90 cm | 0.18 |
21.3 | 150 |
1.8/15 | 4400 |
| 1.24 - 1.70 | 20 cm | 0.0071 | 0.8 | 30 |
0.11/0.35 | 86 |
| 4.5 - 5.0 | 6 cm | 0.0069 | 0.8 | 9 |
0.002 | 3.6 |
| 8.1 - 8.8 | 3.6 cm | 0.0057 | 0.6 | 5.4 |
0.001 | 0.89 |
| 14.6 - 15.3 | 2 cm | |
|
|
|
|
| 22.0 - 24.0 | 1.3 cm | 0.013 |
1.5 | 2 |
0.00001 | -- |
| 40.0 - 50.0 | 0.7 cm | 0.032 |
3.8 | 1 |
-- | -- |
All sensitivity calculations assume 43 MHz bandwidth per IF, (except for P-band and 4-band, where 3.125 MHz and 0.78 MHz are used), two IF pairs (four IFs), natural weighting, and an elevation of 45 degrees. Five VLA and 20 EVLA antennas are assumed, as will be the case in the period near the middle of 2009; since there will be only a few VLA antennas remaining with 2 cm installed, most information regarding this band has been removed. EVLA antennas are assumed to have the same performance as VLA antennas, although they will be significantly better at some bands (particularly at 6 cm = C band); since we mostly have ``transition'' EVLA receivers at present, the final system temperatures are not yet available. Performance will degrade for large zenith angles at high frequencies and for sources close to the galactic plane at low frequencies. The confusion limits (see Condon 2002, ASP Conf. 278, p. 155) for C configuration are approximately a factor of 10 less than those tabulated above for D configuration.
Footnotes:
For a comparison, Table 6 gives the sensitivity goals of the EVLA, including the expectations based on achieved values for those receivers that already have been implemented.
| Band |
Band | Cont. Sens. | Line Sens. | ||
| Code | req. | actual | |||
| (GHz) | (Jy) | (Jy) | ( |
(mJy) | |
| 1 - 2 | L | 325 | 335 | 1.6 | 0.5 |
| 2 - 4 | S | 235 | TBD | TBD | TBD |
| 4 - 8 | C | 245 | 250 | 0.5 | 0.2 |
| 8 - 12 | X | 300 | TBD | TBD | TBD |
| 12 - 18 | Ku | 385 | TBD | TBD | TBD |
| 18 - 26.5 | K | 650 | 450 | 0.6 | 0.2 |
| 26.5 - 40 | Ka | 760 | 675 | 0.85 | 0.2 |
| 40 - 50 | Q | 1200 | 1400 |
1.8 | 0.4 |
Footnotes:
In general, the expected point-source rms noise in mJy on an output image, for natural weighting, can be calculated with the following formula:
| (1) |
where
is the number of antennas,
is
the total on-source integration time in hours,
is
the effective continuum bandwidth or spectral-line channel width in
MHz, and
is the number of IFs (from 1 to 4) or spectral line
channels (from 1 to 512) which will be combined in the output
image2.
is a
system constant, equal to 1000, 50, 8.0, 7.8, 6.6, 27, 14, and
353 for 4, P,
L, C, X, U, K, and Q bands respectively. This constant
also can
be expressed in terms of system temperature and efficiency as:
| (2) |
It is important to note that these listed sensitivities are calculated from data taken in optimum conditions. For many bands, the system temperature and gain are significant functions of elevation and weather conditions (see next section).
A useful alternate form of the point-source sensitivity equation is
| (3) |
The beam-averaged brightness temperature measured by a given array
depends on the synthesized beam, and is related to the flux density by
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| Band | 0.9 x Nominal | 0.5 x Nominal | Extreme Range |
| 90 cm | 305 - 337 MHz | 303 - 342 MHz | 298 - 345 MHz |
| 20 cm | 1240 - 1700 MHz | 1170 - 1740 MHz | 1150 - 1750 MHz |
| 6 cm | 4500 - 5000 MHz | 4250 - 5100 MHz | 4200 - 5100 MHz |
| 3.6 cm | 8080 - 8750 MHz | 7550 - 9050 MHz | 6800 - 9600 MHz |
| 2 cm | 14650 - 15325 MHz | 14250 - 15700 MHz | 13500 - 16300 MHz |
| 1.3 cm | 21200 - 25200 MHz | 20600 - 25200 MHz | 20400 - 25500 MHz |
| 0.7 cm | 40500 - 44500 MHz | 39000 - 47500 MHz | 38000 - 51000 MHz |
For observers interested in HI in galaxies, a number of interest
is the sensitivity of the observation to the HI mass. This is
given by van Gorkom et al. (1986; AJ, 91, 791):
| (5) |
The sensitivity varies across each observing band. Table 7 gives the frequency ranges for each band at which the sensitivity degrades by 10% and by a factor of two for the VLA receivers. Also included are the maximum ranges over which the VLA receivers remain operative. At these extreme ends, the system sensitivity is typically 10 to 100 times worse than at band center. Furthermore, not all antennas will operate at these frequencies. For similar information for EVLA antennas, see Section 3.3. In Figure 1, we show the system temperature and number of operable antennas plotted as a function for frequency for K and Q bands.4 For all bands, consider consulting a VLA staff scientist if you wish to observe near the band edges.
In view of the importance of observations at the lower edge of the 20-cm band for studies of red-shifted HI, some special words are appropriate to describe the performance of the VLA receivers at frequencies below 1250 MHz. The roll-off of this band at the low frequency edge is very gentle, and useful observations at 1155 MHz and lower have been made. However, not all frequencies can be tuned, as tests have shown there are four 10 MHz wide `notches', centered at 1212, 1182, 1162 and 1150 MHz, within which the array can take no useful data. These notches exist in both RR and LL correlations but vary in shape and central frequency from antenna to antenna. A plot of the relative sensitivity between 1150 and 1250 MHz for the VLA receivers is shown in Figure 2.
![]() |
The sensitivity at the low frequency bands (90 cm and 400 cm) is difficult to parameterize. There are two important effects which limit the sensitivity.