The VLA and VLBA, taken separately, leave the 40-to-400 km
range of baselines relatively unsampled. This ``coverage gap''
obstructs work on many astrophysical problems. For example, the VLA
now images well down to about 
FWHM at 
2cm and
proportionally lower resolution at longer wavelengths. The VLBA images
up to a few milli-arcsecond at 2cm. They fail to meet by about
a
factor of ten in resolution at any frequency. The missing regime is
now of great interest to AGN jet dynamics, observations of galactic
X-ray transients, of circumstellar masers, protoplanetary disks,
and of supernova remnants in external galaxies.
It is already common to include one VLA antenna in VLBA observing programs. This is a small step in the right direction, but we need many more baselines in the ``gap'' to image the full range of interesting phenomena satisfactorily.
Figure:
(upper left) The supernova remnant Cassiopeia A, rescaled to a
distance of 800 kpc (M31), at 8 milli-arcseconds resolution.
(upper right) An image obtained by sampling this
structure with the coverage of the
present VLBA alone at 21cm and 30declination.
Only the most compact emission
is represented at all; the diffuse ring and ``plateau"
emission are resolved out and scattered into large-scale
fringes.
(lower left) An image obtained by sampling this structure
with the coverage of the present VLBA and one VLA antenna
at 21cm and 30declination.
The more diffuse emission is detected, but its
brightness distribution is poorly represented.
(lower right) An image obtained with the coverage of the
array shown in Figure 2.7, with four new antennas
and one VLA antenna added to the VLBA
at 21cm and 30declination.
All of the major emission is now correctly
located. For this specific example, the image obtained with eight
new antennas as shown in Figure 2.8 is
only marginally better.
Figure 4.1 illustrates how the new A+ configuration antennas could improve image quality by providing short-baseline coverage in VLBA observations.