Resolution

The VLA's resolution is generally diffraction-limited, and thus is set by the array configuration and frequency of observation. It is important to be aware that a synthesis array is `blind' to structures on angular scales both smaller and larger than the range of fringe spacings given by the antenna distribution. For the former limitation, the VLA acts like any single antenna - structures smaller than the diffraction limit ( $\theta \sim \lambda/D$) are broadened to the resolution of the antenna. The latter limitation is unique to interferometers - it means that structures on angular scales significantly larger than the fringe spacing formed by the shortest baseline are not measured. No subsequent processing can fully recover this missing information, which can only be obtained by observing in a smaller array configuration, using the mosaicing method, or with an instrument (such as a large single antenna) which provides this information.

Table 2 summarizes the relevant information. This table shows the maximum and minimum antenna separations, the approximate synthesized beam size (full width at half-power), and the scale at which severe attenuation of large scale structure occurs.

A project with the goal of doubling the longest baseline available in the A configuration by establishing a real-time fiber optic link between the VLA and the VLBA antenna at Pie Town has been completed. This link has been operational for observations taking place during A configuration. Due to the pressures of EVLA commissioning, the Pie Town link will not be offered during the A configuration session in 2007, and is unlikely to be offered in the future until/unless the link is upgraded to be compatible with EVLA characteristics.

Table 2: Configuration Properties

Configuration	A	B	C	D
$B_{\rm max}$(km$^1$)	36.4	11.4	3.4	1.03
$B_{\rm min}$(km$^1$)	0.68	0.21	0.035$^5$	0.035
	Synthesized Beamwidth $\theta_{\rm HPBW}$(arcsec)$^{1,2,3}$
400 cm	24.0	80.0	260.0	850.0
90 cm	6.0	17.0	56.0	200.0
20 cm	1.4	3.9	12.5	44.0
6 cm	0.4	1.2	3.9	14.0
3.6 cm	0.24	0.7	2.3	8.4
2 cm	0.14	0.4	1.2	3.9
1.3 cm	0.08	0.3	0.9	2.8
0.7 cm	0.05	0.15	0.47	1.5
	Largest Angular Scale $\theta_{\rm LAS}$ (arcsec)$^{1,4}$
400 cm	800.0	2200.0	20000.0	20000.0
90 cm	170.0	540.0	4200.0	4200.0
20 cm	38.0	120.0	900.0	900.0
6 cm	10.0	36.0	300.0	300.0
3.6 cm	7.0	20.0	180.0	180.0
2 cm	4.0	12.0	90.0	90.0
1.3 cm	2.0	7.0	60.0	60.0
0.7 cm	1.3	4.3	43.0	43.0

These estimates of the synthesized beamwidth are for a uniformly weighted, untapered map produced from a full 12 hour synthesis observations of a source which passes near the zenith.

Footnotes:

1. $B_{\rm max}$ is the maximum antenna separation, $B_{\rm min}$ is the minimum antenna separation, $\theta_{\rm HPBW}$ is the synthesized beam width (FWHM), and $\theta_{\rm LAS}$ is the largest scale structure `visible' to the array.
2. The listed resolutions are appropriate for sources with declinations between $-$15 and 75 degrees. For sources outside this range, the extended north arm hybrid configurations (BnA, CnB, DnC) should be used, and will provide resolutions similar to the smaller configuration of the hybrid, except for declinations south of $-$30. No double-extended north arm hybrid configuration (e.g., CnA, or DnB) is provided.
3. The approximate resolution for a naturally weighted map is about 1.5 times the numbers listed for $\theta_{\rm HPBW}$. The values for snapshots are about 1.3 times the listed values.
4. The largest angular scale structure is that which can be imaged reasonably well in full synthesis observations. For single snapshot observations the quoted numbers should be divided by two.
5. The standard C configuration has been replaced by a slightly modified one, formerly known as CS, wherein an antenna from the middle of the north arm has been moved to the central pad `N1'. This results in improved imaging for extended objects, but will degrade snapshot performance. Although the minimum spacing is the same as in D configuration, the surface brightness sensitivity to extended structure is considerably inferior to that of the D configuration (but considerably better than standard C configuration).