Figure: (left) The supernova remnant Cassiopeia A, rescaled
to a distance of 800 kpc, (right) as imaged with the present
VLA in the A configuration at 
5cm, (FWHM 
) giving only a rudimentary indication of
shell structure.
We illustrate the improved imaging capability of the expanded
coverage in the A+ configuration by considering an observation of a
``twin'' of the Cassiopeia A supernova remnant at the distance of
about 800 kpc, as if the remnant were in M31. The angular extent of
the remnant would be about 
, and the total flux density at
5cm (the center of the proposed new 4-8 GHz observing band)
would be 12 mJy.
The left panel of Figure 2.9 shows how this object should appear at 35 milli-arcsecond FWHM resolution in the absence of noise -- a suitably scaled model of Cassiopeia A has simply been convolved to the appropriate resolution.
The right panel of Figure 2.9 shows the image that would be obtained using the present VLA A configuration, with the noise appropriate for the present VLA C Band system. The shell structure of the remnant is barely hinted at, and no details of the knot structure are seen.
Figure: Cassiopeia A rescaled to 800 kpc distance, as imaged at
5cm
with
(left) the proposed 6-element A+ configuration (four new antennas),
(right) the proposed ten-element A+ configuration (eight new antennas).
Contours are shown at -1.4, 1.4, 3.5, 7, 14, 21, 28, 35 and 42 \
per
beam.
The left panel of Figure 2.10 shows the image obtained
using the A+ configuration with four new antennas as proposed in the
in 1995 (Figure 2.7). The right panel shows the
image obtained using the A+ configuration with eight new antennas as
shown in Figure 2.8. In both cases the FWHM of the
synthesized beam is 
milli-arcseconds, revealing many details
of the shell and filament structure. Noise appropriate for the
upgraded VLA was added to the model during these imaging simulations.
Figure:
The differences between the model image and the reconstructed images
of the scaled Cassiopeia A remnant shown in Figure 2.9.
The rms residuals within the source for the six-element A+
configuration (four new antennas) are about three times larger than
for the ten-element A+ configuration (eight new antennas), and the
largest localized errors are ten times the off-source ``noise''.
At first sight, the image obtained with the six-element A+ configuration (four new antennas) contains much of the fine scale information present in the model. It is, however still only a first-order representation of the model brightness distribution. Although the off-source noise is well-behaved in this image, the on-source information is contaminated by ``lumpy" artifacts arising from the paucity of baseline coverage and limited sensitivity in the outer plane.
These artifacts can clearly be seen in Figure 2.11, which shows the difference between the reconstructed images and the original model at the same resolution and with the same intensity scale. There are localized on-source errors up to ten times the off-source ``noise'' in the image obtained with the six-element A+ configuration. These could severely compromise work on the fine structure and time evolution of the supernova remnant. Local errors in the intensities in this image are as large as 30%, even where the ratio of the signal to the rms off-source noise is as high as 20:1. The peak residuals inside the source are seven times the rms noise off-source.
This demonstrates that the number of antennas that should be placed in the ``second ring'' around the VLA depends strongly on the target image quality. In this example, an eight-antenna extension reduces the differences between the image and model almost to the level of the off-source noise, while the image obtained from the four-antenna extension would be much less useful for quantitative work on the fine structure in the remnant. It is, of course, difficult to predict how specific image imperfections will affect the astrophysical goals of particular projects. These simulations do, however, suggest that the number of new antennas added for the A+ configuration must not be less than four, and that an array with as many as eight new antennas is needed for good image fidelity.