VLA Expansion Project: June 1998 Design Review
(revised February 1999)
The Very Large Array has transformed many areas of radio astronomy
with a powerful combination of angular resolution and brightness
sensitivity. Even so, twenty years of research have also exposed many
ways in which the VLA's limitations direct our observations.
Depending on the field of study, we may be biased to objects that are
unusually luminous examples of their class, or unusually nearby, or
not too extended, or in a particular redshift range, because of
constraints on the VLA design that were accepted in the 1970's. Many
of these constraints can be greatly relaxed today.
The scientific capabilities of the VLA can again be transformed by
returning it to the state of the art in sensitivity, frequency
coverage, angular and spectral resolution. An expanded VLA could
be over one hundred times faster at high frequencies, several times
more frequency-agile, and one hundred times better at resolving
details at all frequencies, for substantially less than the
replacement cost of the instrument.
Many important observing programs are limited (or prohibited!) by
sensitivity. The VLA's intermediate-frequency transmission system and
correlator were designed with a maximum total bandwidth of 200 MHz.
This limits the sensitivity, particularly at frequencies above 2 GHz,
where wider bandwidths are permitted by the antennas, the feeds, and
the level of interference. The higher frequencies are very attractive
for many fields of research, including studies of the Sun and solar
system objects, of thermal (stellar and circumstellar) sources in the
Milky Way, of young supernovae in other galaxies, of polarimetry of
the jets from active galactic nuclei, of the dense magnetized media in
gas-rich clusters of galaxies, and of ``radio-quiet'' source
populations at high redshifts. Fiber optic signal transmission,
combined with a new correlator, can increase the maximum total
bandwidth to 16 GHz, enabling a huge improvement in the VLA's
capability in many astrophysical arenas.
Similarly, the VLA correlator is based on a custom ECL circuit that
was the state of the art in the 1970's. The correlator limits the
number of frequency channels and frequency resolution in ways that are
particularly onerous for spectral studies at high frequencies, and for
wide-field surveys at low frequencies. Modern designs allow much
greater spectral resolution, wider bandwidths, and increased
flexibility. A new state-of-the-art correlator would, for example,
allow complete surveys of the neutral hydrogen gas in the Universe out
to 
0.2, avoiding bias in present surveys of the local
structure that are based on cataloging galaxies. We could also
independently and simultaneously observe multiple spectral transitions
in one or two frequency bands. An expanded correlator would let us
exploit the sensitivity of broad-band systems to image wide fields of
view in the continuum, by using bandwidth synthesis to image
nearby galaxies and to inventory their stellar and interstellar
emissions with less bias toward the brightest or most compact
features.
The VLA antennas can be used up to roughly 50 GHz, and we have
experience
with holography and reference pointing to optimize their
high-frequency performance. These techniques justify outfitting the VLA
more
fully for frequencies above 20 GHz, where we can explore thermal
processes around galactic stars, image the structures of
protoplanetary disks, detect young supernovae in nearby galaxies, and
study CO and other molecules in galaxies at high redshifts.
The VLA's maximum baseline of 35 km sets an angular resolution
limit at any frequency. The VLBA's inner baseline scale (about
300 km) similarly sets a largest angular scale that it can image
efficiently at any frequency. The disparity, or ``gap'', between these
two scales leaves a wide range of interesting astrophysical phenomena,
including stellar winds, star-forming regions, and the inner regions
of bright extragalactic jets, inaccessible to imaging by either
instrument. These phenomena are, in effect, unresolved to the VLA but
over-resolved by the VLBA, even when existing VLA antennas are used in
VLBA observations. In many cases, the VLA has the sensitivity, but
not the resolution, to make useful observations -- a situation which
will be much more general if we upgrade only its sensitivity. The
``coverage gap'' also seriously restricts the range of observing
frequencies at which angular resolutions between one arcsecond and
tens of milli-arcseconds are available. This frequency ``inagility''
at fixed angular resolution limits our ability to explore the
frequency dependence (and hence the physics) of phenomena with angular
scales of tens of milli-arcseconds.
Bridging the ``VLA-VLBA'' imaging gap with new antennas in New Mexico
and Arizona will greatly enhance the scientific productivity of both
major centimeter-wave instruments. Operating the
``intermediate-baseline'' antennas as part of either instrument would
let us match spatial filters to the needs of a wide range of
astrophysical studies that cannot be pursued satisfactorily with
either instrument alone. Furthermore, the intermediate baseline
antennas would themselves perform superbly as a standalone array,
giving permanent, high sensitivity, high fidelity imaging capability
on small angular scale phenomena such as novae, GRBs, and expanding
galactic jets.
Finally, some planned VLA subsystems were never built for lack of
development (infrastructure) funding, although there are strong
scientific reasons for adding them. For example, the 2.4 GHz system,
initially of interest for planetary radar and for Faraday depth
studies of galactic and extragalactic sources, is now of interest to
all continuum projects because RFI from mobile communications systems
is proliferating at lower frequencies.
It has long been clear that the VLA could be transformed into a much
more powerful tool for astrophysics at only a modest cost relative to
the original investment (in today's dollars). Ideally, its
construction would have been followed by timely upgrades of the
technology in its infrastructure, allowing a steady increase in its
scientific capabilities. In fact, the NRAO's operating budget has not
kept pace with inflation since the construction of the VLA, so only a
few high-priority, but still incremental, improvements have been
possible, and several of these have been done with non-NSF funds. The
imbalance between our technical ability to increase the VLA's
scientific productivity and the ability to fund even modest
improvements to the instrument has grown steadily. The accumulated
imbalance can now be addressed only by pervasive upgrades to the VLA
technology.
In January 1995, the NRAO hosted a workshop at which VLA users and
NRAO staff reviewed technical aspects and scientific benefits of an
upgraded VLA. The conclusions of six topical working groups (on Solar
System research, on galactic astronomy (2 groups), on extragalactic
astronomy (2 groups) and on technical issues) during and after this
meeting were published as the
in July 1995. Shortly thereafter, the NRAO formally organized a VLA
Upgrade Project to prepare a more detailed design document summarizing
the technical and engineering goals of a , the technical means
and costs to achieve them, and an ongoing review of the scientific
benefits to be obtained.
The general goal of the is to improve, by at least an
order of magnitude, every key instrumental characteristic of the VLA.
The resulting scientific benefits are enormous, as the existing
infrastructure and sound basic design of the VLA now let us exploit
modern technologies at modest cost.
It is important to note that much of this greatly enhanced scientific
potential can be achieved by using newer, but well established,
well understood, technologies. And because MMA development will
implement many of these technologies, their incorporation in the
can be done at very modest extra cost. Furthermore, the
operations cost of the Expanded VLA should be only slightly increased
over current levels. Indeed, for the improvements at the VLA site, we
expect no additional cost in operations. The only anticipated increase
will be due to the operation of the new antennas.
The
identified scientific motivations for improving the VLA in four major
areas:
- Sensitivity.
- Frequency coverage.
- Spectral line resolution, bandwidth, and flexibility.
- Angular resolution.
The general goal of the is to improve, by at least a
factor of 10, the performance of the VLA in all four areas.
Ongoing development of the upgrade concept has demonstrated
that improvements of a factor of ten or more over current capabilities
are achievable in all four key areas. We define the following as
primary performance goals of the :
- Sensitivity The current sensitivity limit (the expected rms
in an image made with 12 hours' data, natural weighting, all available
antennas, and the maximum available bandwidth) for the VLA is
approximately 6 at the most sensitive bands - 1.4 GHz,
4.9 GHz, and 8.4 GHz. Improvements in bandwidth, receiver design, and
antenna efficiency can greatly improve the sensitivity at all bands,
especially at the higher frequencies. At the lower frequencies, the
sensitivity limit is reached in practice only over a limited area of
the primary beam at high angular resolution, because of the effects of
bandwidth smearing; an upgraded correlator can relieve this
restriction. We therefore define as a goal:
The means by which this sensitivity will be obtained will also give
significant improvement at other frequencies - typically by a factor
of five at frequencies below 2 GHz, and a factor of ten to thirty for
the 40 to
50 GHz band. Improved sensitivity also brings improved
calibration (and self-calibration) quality because the averaging
time needed to reach given phase and amplitude noise is reduced,
and because more (weaker) sources can be used for phase-referencing.
- Frequency Coverage The current VLA has very limited
frequency coverage - targeting only the major centimeter-wavelength
spectral lines (, OH, , ) and selected narrow
continuum bands. Many considerations, based both on continuum and
spectral line observing, now argue for a much expanded frequency
tunability. We therefore define as a goal:
This frequency range is that which can be accessed from the Cassegrain
secondary focus ring. We would like to obtain similar frequency
availability at lower frequencies as well, although not at the cost of
diminishing the array's capabilities at higher frequencies. We thus
define a secondary goal:
which must be accomplished from the prime focus.
- Spectral Line Resolution, Bandwidth, and Flexibility The
current correlator severely restricts the science from the
VLA. A modern correlator is critically needed to keep pace with the
array's current, and future observing capabilities. A primary goal
is:
but note that these are not all required simultaneously.
- Angular Resolution The detailed
the broad-based scientific benefits from increasing the VLA's
resolution. Because the objects we wish
to observe are complex, it is not sufficient to add a sparse
sampling of intermediate-length baselines. To maximize the scientific
return, we define as a goal:
These improvements would make a ``new VLA" that would
continue to set the world standard for radio astronomical imaging from
meter wavelengths to long millimeter wavelengths for another
generation.
The elements of the can be categorized in two broad groups:
an Ultra-Sensitive Array, with many separable improvements to the
on-site capabilities, and an A+ Configuration, an expansion
of the array by a factor of about ten with antennas that will also
be cross-linked to the VLBA. The combination of these enhancements
will yield an instrument with many fundamentally new capabilities,
as well as significantly improving the imaging and scaled-array
performance of the VLBA.
These enhancements can be made entirely within the existing
infrastructure
at the VLA Site:
- New Cassegrain receivers: lower noise temperatures and
much wider bandwidth performance (up to 8 GHz in each polarization
channel) in existing bands; add 2.4 GHz and 33 GHz bands; complete the
outfitting at the 40-50 GHz band; extend the 1.4 GHz band downwards to
1.1 GHz or lower.
- New prime focus receivers: rebuild the focus-rotation
mount so that the subreflector can be removed, and add at least three
low-frequency feeds at the prime focus.
- A fiber-optic data transmission system to transmit the
broad-band signals and monitor data, replacing the original waveguide.
- A new correlator, able to support at least 40 antennas,
with up to 8 GHz
in each polarization and up to 8,000 spectral channels per baseline per
polarization product, and the ability to independently process up to 8
different frequencies from one or two frequency bands.
- Improved antenna performance by adjusting panels for
higher efficiency and developing techniques to permit better pointing.
- New antenna stations for a ``super-compact'' E
configuration to enable fast mosaicing of large fields.
- New On-Line Computing, monitor/control, and archiving
capabilities to permit much more flexible observing modes and user
interface to the array.
Figure: The current (+) and predicted (o) continuum (left) and
spectral line (right) sensitivity of the VLA after the upgrade. For
both panels, a 12-hour observation with 27 antennas and with the
efficiencies and system temperatures listed in Table
2.3 are assumed. The continuum sensitivity assumes
bandwidths given in Table
2.3,
while the line sensitivity is based on a bandwidth equivalent to 1
. (Note the different vertical scales.) For the A+
configuration and 37 antennas, the sensitivity would be improved by a
further factor of 1.33. To illustrate the relative
sensitivities of the proposed new bands for nonthermal and thermal
objects, the left panel also shows the slopes of a typical
synchrotron spectrum (dashed) and of an optically thick thermal
spectrum (dotted). The vertical placement of these spectra is
arbitrary.
The continuum sensitivity will improve by more than an order of
magnitude in some bands. Figure 2.1 shows the
current and expected sensitivity for both line and continuum
observations, and demonstrates how these various improvements will
impact all observing frequencies. Typical spectra for an optically
thin object, and an optically thick thermal object are shown to
demonstrate how the proposed S and Ka bands will be the best for work
on these objects. New and powerful spectral line observations will be
possible and significantly more frequency choices will be available.
Note also that although the sensitivity improvements per spectral line
shown in Figure 2.1 are modest below 10 GHz, the
proposed new correlator will allow several different transitions to be
observed simultaneously. Some multi-transition spectral-line
experiments will be more sensitive because longer integration times
are available per line.
These enhancements involve the development of new infrastructure at
locations away from the VLA Site:
- New antennas to provide now-unavailable baselines
between those in the VLA and in the VLBA. To maintain VLA-like
imaging fidelity, of order eight new stations will be
be required.
- Fiber-optic links between the VLA and the inner VLBA
antennas, and between the VLA and the proposed new antennas, to
provide for real-time data correlation with the upgraded correlator.
When the VLA antennas are not in the A-configuration, the new
antennas can either be part of an expanded VLBA, (where the eight new
antennas will enormously increase the imaging capability of that
array), or they can run as a standalone array with excellent
sensitivity and imaging performance on a scale ten times finer than
the VLA can currently achieve in its A-configuration.
Linkages to the innermost VLBA antennas and the new antennas will
increase the maximum angular resolution by a factor of about ten. The
sensitivity increases will allow this increased angular resolution to
be exploited fully when observing a wide (and in many cases for the
first time, representative) variety of thermal and nonthermal objects,
both galactic and extragalactic.
The primary goals of providing continuous frequency coverage and
sub- sensitivity require many changes to the antennas and
receiver ensemble. At the antennas, the project involves: i)
improving receivers at existing observing bands; ii) adding new
observing bands; iii) modifying the quadrupod legs and focus-rotation
mount system to permit access to the prime focus; and, iv) modifying
the antenna surface for improved operation.
The VLA receivers have been upgraded gradually since the early 1980s.
Initially, better low-noise amplifiers were used in existing
receivers. More recent systems have used the VLBA design, in which
the receiver is attached directly to the feed and the polarizer is
cooled in the cryogenic dewar. This design reduces the noise
contribution from the polarizer and eliminates long, ambient
temperature waveguide runs that added to the system temperature.
The ``VLBA-style'' receivers are now used at 1.4, 8.4, and 40-50 GHz.
These receivers will likely remain, with the only changes being new,
modern amplifiers, and improved, wider-band polarizers and feed horns.
The greatest improvement in system temperature can be made in the 5,
15, and 23 GHz bands using the VLBA-style receivers and modern HFET
amplifiers. Completely new receivers will be built for these bands,
and should reduce the system temperatures by up to a factor of three.
The new receivers will also provide much wider bandwidth capability
(needed for continuum sensitivity) and will tune over a wider
frequency range (to include spectral lines, methanol, whose
astrophysical significance was unknown when the VLA was built, and
permit observing of redshifted molecular lines from distant galaxies).
Current plans call for bandwidth ratios of order 1.5 to 2.0 in all
bands.
The improvements at the 23 GHz and 40-50 GHz bands are already
funded, and will be completed late in 2001.
Table 2.1 summarizes the proposed Cassegrain
receiver suite. Figure 2.2 shows the arrangement of
the feeds around the secondary focus.
|
Band | Range | Bandwidth | Polarization | Status | |
|
| (GHz) | Ratio | | | |
| L | 1.10-2.00 | 1.82 | Circular/Linear? | New Feed | |
|
S | 2.00-4.00 | 2.0 | Circular/Linear? | New | |
|
C | 4.00-8.00 | 2.0 | Circular/Linear? | Upgrade | |
|
X | 8.00-12.0 | 1.5 | Circular | New Feed | |
|
Ku | 12.00-18.00 | 1.50 | Circular | Upgrade | |
|
K | 18.00-26.50 | 1.47 | Circular | Upgrade | |
|
Ka | 26.50-40.00 | 1.51 | Circular | New | |
|
Q | 40.00-50.00 | 1.25 | Circular | Complete | |
Table 2.1: Proposed VLA Cassegrain Observing Bands
Figure 2.2: A possible arrangement of eight
Cassegrain feeds around the secondary focus circle.
Two new receiver systems will be added at the Cassegrain focus:
2.4 GHz and 33 GHz. A stand-alone 2.4 GHz feed will fit on the feed
ring and will give higher performance than,
an 1.4 GHz/2.4 GHz dual-band feed such as that on the
Australia Telescope Compact Array. The 2.4 GHz band offers the highest
sensitivity for studies of optically thin synchrotron-emitting
objects, as shown in Figure 2.1 by the proximity
of the sensitivity curve to the dashed synchrotron-spectrum line. It
will also let the VLA participate in VLBA observations, and in
bistatic planetary radar observations with Arecibo Observatory and
Goldstone, at this wavelength.
The 33-GHz band is potentially the most sensitive band for detecting
and imaging thermal objects with the VLA (as shown in
Figure 2.1 by the proximity of the sensitivity
curve to the dotted thermal-spectrum line). It is a band rich in
molecular transitions, and is of interest to studies of optically
thick (very compact) synchrotron sources.
It is advantageous to use circular polarization in the signal
transmission, as amplitude and phase variations between the two
oppositely polarized channels then have less effect on the measurement
of linear polarization. The five high frequency bands, each covering
a bandwidth ratio of 1.5:1, will use polarizers of a design now being
tested with the new K-Band VLA receivers. These devices are large,
and therefore unsuitable for the three lowest bands, where we will use
polarizers employing a 3 dB hybrid design that are expected to provide
sufficiently pure conversion from linear to circular polarization.
Plans for new prime focus receiver systems are less well defined. The
VLA's current low frequency capability consists of a fixed 300-340 MHz
band and a dismountable, trial 73.4 - 74.2 MHz band. Both use
crossed, on-axis dipole feeds with the (curved) subreflector as a
backplane - a very low efficiency arrangement. The scientific
potential of these lower frequency systems is high, so it is desirable
to replace both systems with an arrangement which would permit
continuous frequency coverage below 1 GHz from the prime focus with
higher efficiency if this can be done without degrading antenna
performance at high frequencies.
Figure 2.3: A sketch of the modified quadrupod and rotating feed assembly
at the prime focus. The horizontal quadrupod legs would be replaced
with narrow tubes, allowing the subreflector to be rotated away from
the prime focus about a vertical axis and exchanged for any of four
potential UHF feeds (shown in outline).
The current plan is to replace each of the horizontal quadrupod legs
with a pair of thinner tubes which would permit rotation of the
subreflector between them. Three or more low frequency feeds could
then be mounted as part of the entire rotating package to permit
excellent low frequency coverage. One of these could, for example, be
a broad-band UHF (
150 to 1000 MHz) system. A suggested receiver
suite is given in Table 2.2. A sketch of the
proposed new feed-rotation system is shown in
Figure 2.3
|
Band | Range | Bandwidth |
|
| (MHz) | Ratio |
| U1 | 700 - 1100 | 1.57 |
|
U2 | 450-700 | 1.56 |
|
U3 | 300-450 | 1.50 |
|
U4 | 200-300 | 1.50 |
Table 2.2: Potential Prime Focus Observing Bands
Figure 2.1 compares the continuum and line
sensitivity of the current instrument to that we expect to achieve
with the . We assume a maximum usable bandwidth with RFI excision
at the lower frequencies, and add an atmospheric and galactic
contribution to the system temperature, assuming a typically dry
atmosphere, and (for the low frequency cases), observations away from
the galactic plane. The system temperature assumes new, modern
receivers, employing cooled polarizers (the ``VLBA design'') and new,
wide-band feed horns. The values for the receiver and system
temperatures, efficiency, and bandwidth used for the calculation of
the upgraded sensitivity are shown in Table 2.3.
The listed bandwidth is the sum of the orthogonal polarizations, and
the quoted sensitivity is 
for a 12-hr observation.
To transmit up to 16 GHz of total bandwidth from each antenna, we will
use optical fiber links to all of the VLA stations, to the nearby VLBA
antennas and to the new antennas in the A+ configuration
(§2.7). Separate fibers will carry the LO reference signal
and the wide-band IF signal. Between four and six single mode fibers
will run to each antenna station. Although low temperature
coefficient fiber will be used on runs exposed to ambient temperature,
a round trip phase correction system probably will still be needed.
The detailed design will be based on the MMA development of a similar
system and on experience gained with the VLA-to-Pie-Town link project.
An important ingredient for using the more
remote antennas with the VLA will be to explore the use of
commercial digital networks to transmit the IF signals.
Table 2.4 shows the capacity of the current VLA
correlator, whose properties were specified in the mid-1970's.
One measure of correlator size is the number of multiplications
required per second, which scales as:
(Number of antennas)
(Maximum value of: (Bandwidth Number of channels at that
bandwidth)).
Table 2.5 compares this figure of merit for
existing or proposed synthesis-array correlators.
|
Bandwidth | Channels | Separation |
|
(MHz) | | (kHz) |
| 50 | 16 | 3125 |
|
25 | 32 | 781.25 |
|
12.5 | 64 | 195.313 |
|
6.25 | 128 | 48.828 |
|
3.125 | 256 | 12.207 |
|
1.5625 | 512 | 3.052 |
|
0.78125 | 512 | 1.526 |
|
0.195313 | 512 | 0.381 |
Table 2.4: Maximum Channels in Current VLA Spectral Line Modes
|
Correlator | Antennas N | Bandwidth B | Channels C | ``Size" |
|
| | (MHz) | | ( ) |
| VLA now | 27 | 50 | 16 | 5.8 E 5 |
|
ATCA | 6 | 128 | 128 | 5.9 E 5 |
|
VLBA | 20 | 128 | 128 | 6.6 E 6 |
|
WSRT | 20 | 160 | 512 | 2.1 E 7 |
|
SMA | 6 | 1968 | 3072 | 2.2 E 8 |
|
MMA | 40 | 2000 | 1024 | 3.3 E 9 |
Table 2.5: Sizes of Synthesis Correlators
The scientific drivers for a new VLA correlator have been reviewed by
Rupen (1997) in VLA Upgrade Memo #8.
A new, and much larger, correlator is clearly required because (a) the
maximum of 16 channels at 50 MHz bandwidth is inadequate for most
spectroscopy, (b) much wider bandwidths (8 GHz per polarization,
versus the current 100 MHz) are now proposed for the higher observing
frequencies, and (c) the number of antennas whose outputs must be
correlated will increase to forty or more from the current
twenty-seven as we pursue increased angular resolution.
The new correlator must also provide flexibility in choice of
bandwidths and frequency resolution (numbers of channels) that will
allow observers to take full advantage of the improved sensitivity and
broad-band capability of the instrument. This broad requirement of
flexibility encompasses several other desiderata, notably high
linearity, so that strong interfering signals can be removed in
post-processing without corrupting nearby channels; the ability to
recognize and remove time-variable interference on sub-integration
timescales; the availability of temporal gating for pulsar
observations and to blank out pulsed RFI. It also requires support
for up to five independent subarrays, and most importantly the ability
to use different frequency resolutions for each of several
independently-tunable IF's (sub-bands). From the point of view of
spectroscopy, a reasonable maximum requirement for the is 8192
channels (summing 4096 in each of RR and LL) over 125 MHz. As this
replicates the MMA correlator's capacity in this key quantity, one
option that may be attractive logistically is to duplicate the
proposed MMA correlator (Escoffier 1997; Rupen and Escoffier 1998).
This correlator is described in more detail elsewhere, and we discuss
here only how it would appear to the observer. The 16 GHz
bandwidth (8 GHz in each of two polarizations) available for each
antenna is presented to the correlator in four pairs of baseband
signals, each with a maximum bandwidth of 2 GHz. The possible modes
for a single baseband pair (pair of oppositely polarized signals)
are given in Table 2.6.
Each of the four baseband pairs can operate independently, giving a
great deal of flexibility. Most obviously, one can use the baseband
pairs together to cover a wider bandwidth at the same frequency
resolution: the widest-band mode uses all four baseband pairs to cover
4 2 = 8 GHz with 4 256 = 1024 channels, if only a
single polarization product were desired; or one could cover 4
0.25 = 1 GHz with 4 512 = 2048 channels and full
polarization information (four polarization products), yielding a
channel separation of 490 kHz. One could also use more than one
baseband pair to produce higher frequency resolution for the same
input bandwidth, to provide 32768 (single polarization product)
channels over 15.625 MHz, with a channel separation of 0.48 kHz. For
comparison, the current VLA correlator gives a channel separation of
195 kHz over 12.5 MHz with 64 channels (see
Table 2.4).
More complicated modes are also possible, since each baseband pair is
independent. One might, for instance, use one baseband pair to
produce a 512-point, full polarization spectrum covering a total of
250 MHz; employ another two to cover a total bandwidth of 62.5 MHz at
3.8 kHz resolution, RR only; and use the last baseband pair to ``zoom
in" on 1 MHz of the spectrum with 4096 dual-polarization channels,
giving a resolution of 0.24 kHz. The highest available frequency
resolution is set by the IF/LO system rather than the correlator; if
that system could provide a 32.8 kHz bandwidth to the samplers, the
resulting channel separation could be as little as 1 Hz. Finally,
this correlator design can handle up to four independently-tunable IF
pairs if these are provided by the LO system. Even with only two
independent LO systems, one could observe simultaneously two lines, or
a narrow line and a broad-band continuum.
The MMA correlator requirements do differ from those of the in
several respects, however. For the there is little scientific
case for a contiguous bandwidth greater than 500 MHz, and only
one scientific driver requires more than 16384 channels (full-field,
full-band, full-polarization imaging at L, S, or C Bands in the A+
configuration). Deep continuum imaging experiments would use a total
bandwidth of 16 GHz, as in the MMA, apportioned as two polarizations
of 8 GHz each. As 500 MHz is the widest required individual
bandwidth, sixteen separate input pairs of this width or eight pairs
of 1 GHz would be appropriate for the correlator. For the
the desire to do bistatic radar experiments sets a very small
narrowest channel width of a few Hz, but this need only be available
for about 2048 channels total; the lowest channel width driven by
(extra-solar-system) spectroscopy is about 200 Hz.
The design of the correlator must also address two concerns
driven by the current and expected future ``hostile environment". It
is important to have considerable tuning flexibility to avoid harmful
RFI. This is primarily of concern to designing the LO/IF system: very
strong RFI must be blocked before reaching the correlator. Having
more, but narrower, pairs of inputs to the correlator will, however,
help strategies to avoid ``mid-strength" RFI. As there will always be
some RFI within the bands input to the correlator, a high spectral
dynamic range is also important. This requires more quantization
levels than the MMA's planned 2-bit, 4-level system, especially at the
narrower input bandwidths which would be used at lower frequencies.
A critical aspect of the design will be the ability to assign
different numbers of spectral channels amongst the independently
tunable IF's (``sub-bands''). This is needed for targeting up to 8
different spectral transitions with differing resolutions appropriate
for each, and for permitting RFI avoidance by moving the sub-bands to
cleaner parts of the spectrum with sufficient spectral resolution to
permit efficient removal of weaker RFI. In a similar vein, the gating
ability is needed both for scientific and practical purposes -
observations of pulsars, and potentially, to blank the correlator
against pulsed RFI.
The new correlator must also be designed to be ``VLBA-Enabled''. To
include more distant antennas in the A+ configuration, it must be able
to handle high fringe rates and delays, and include buffering to
account for delays in signal transmission over commercial fiber optic
lines. Ultimately, the correlator could become a 50-station unit,
processing all the current VLA and VLBA antennas in real time, plus as
many as eight new antennas, plus a number of ``foreign'' stations, for
special experiments that require the ultimate in spatial frequency
coverage.
The clearly requires a very large correlator to achieve the
desired scientific productivity, but one that is technically feasible
and in many respects similar to that required for the MMA. Plans for
the development of the MMA and correlators will therefore be
closely coordinated, whether or not the two are ultimately identical.
Figure 2.4: Examples of possible E configurations: the E1 (left)
requires only 9 new
antenna stations and two new rail spurs; the E2 (right)
requires 27 new stations and five or six new spurs.
When the VLA was designed, little emphasis was given to achieving good
sensitivity to low surface brightness features, to imaging of fields
of view wider than its primary beam, or imaging with lower angular
resolution than that provided by the D configuration. Mosaicing had
not been developed and it was believed that, in any case, such issues
were better addressed by large single dishes. It is now recognized
that compact arrays with total power capabilities fill a gap between
the imaging capabilities of conventional interferometer arrays and
those of single dishes.
An ultra-compact E configuration with maximum baseline lengths
of a few hundred meters would allow efficient high-fidelity imaging of
extended, low-surface brightness emission on angular scales larger
than the primary beam. Support of this configuration will require new
antenna stations and rail access to them.
The E1 option is to move the outer three antennas
from each arm of the D configuration to the inner part
of the configuration. This would require nine new stations
and two new rail spurs (Figure 2.4) and provide
a maximum baseline of 500 m. Simulations show that it would
reach a given surface brightness sensitivity twice as fast as the
optimally tapered D configuration.
The E2 option would use 27 new stations and five or six new rail spurs
to provide essentially Nyquist-sampled coverage and a small
grating response. Simulations of the configuration in
Figure 2.4 show that it will reach a given surface
brightness sensitivity five to ten times faster than the D
configuration.
Figure 2.5: Relative sensitivities of the D, E1 and E2 configurations.
Fig. 2.5 compares the relative sensitivities of the
D, E1, and E2 configurations (parameterized as the filling factor of
antennas in the area covered by the array). The E2 configuration is
preferable, but is estimated to cost about four times the E1.
Shadowing is of little concern for either possibility for hour angles
HA 
and source declinations 
. Long tracks are not
needed for good coverage; snapshots at several hour angles will
suffice for many programs. For low declination sources, a hybrid or
``E-south'' array configuration may be needed.
In principle, much of the same capability for imaging
low-surface-brightness emission could be obtained by using array feeds
on a large single dish, the GBT. In practice, the E configuration
would guarantee this capability for all VLA observing frequencies and
simplify mingling of the data with those from other configurations.
An alternative to constructing new rail spurs and VLA antenna stations
might be to develop the E configuration as a test-bed for prototype
antennas for (some versions of) the proposed Square Kilometer Array.
There is a serious gap in coverage between the 35-km longest
baseline of the VLA and the 200-km shortest baseline of the VLBA. This
gap is well illustrated in the frequency-resolution plot shown in
Figure 2.6.
Figure: The frequency-resolution coverage of the VLA (dark) and
VLBA (shaded). The sloping ``VLA-VLBA Gap'' is caused
by the absence of antennas with separations between 35 and 250
km. Much astrophysical analysis depends on
imaging at fixed angular resolution over a wide frequency range,
on vertical lines in this diagram. The ``gap''
seriously restricts interpretative work at resolutions from a
few hundred milli-arcseconds to one arcsecond.
Figure: Possible antenna locations for a six-antenna A+ configuration,
as suggested in the (1995). Four new antennas are added, at
Dusty, Bernardo and Vaughn in New Mexico and at Holbrook, in Arizona.
The absence of imaging capability on scales between the VLA and VLBA
severely limits scientific investigation of a wide range of phenomena.
The gap extends over angular scales of tens to hundreds of
milli-arcseconds - a range critical for studies of stellar emission,
star-formation, radio jets, and gravitational lenses, to name just a
few.
Sub-arc-second resolution observations with the present VLA are
possible only at the higher frequencies where the brightness
sensitivity needed to detect nonthermal phenomena is usually
inadequate because of the source spectra. These frequencies may also
be inappropriate for projects in which Faraday depth effects,
wide-band spectral shapes, or low-frequency spectral lines are needed
to probe source physics at these resolutions. There are also many
instances where studies of normal stellar radio emission will benefit
greatly from increasing both the resolution and sensitivity of the VLA
together at higher frequencies.
The reconfigurability of the VLA (its capacity for ``scaled array''
observations with the same angular resolution at several frequencies)
has been vital to its astrophysical success. This distinctive
capability of the VLA is now present across the whole frequency
range from 330 MHz to 22 GHz only at resolutions between
and 
. No problem that requires higher angular
resolution than and full frequency coverage can now be
tackled in scaled-array mode. This situation improves dramatically -
by a factor of order ten - if we can add data from longer baselines
in an A+ configuration that includes the two innermost VLBA antennas.
Finally, we note that the availability of optical imagery at 
resolution from the Hubble Space Telescope is emphasizing the need for
better coverage of this resolution regime over a full range of radio
frequencies. This resolution falls squarely in the ``uncovered gap''
between the VLA and VLBA at just the frequencies where imaging would
be least corrupted by atmospheric and ionospheric phenomena. The NGST
may also provide complementary near infrared imaging at similar
resolutions.
We therefore plan to bridge this gap by adding new antennas
distributed between 50 and about 400 km from the VLA, and by enabling
some VLA, some VLBA, and all of the new antennas to be used
interchangeably as members of either array.
These enhancements will (a) increase the resolution of the VLA at all
frequencies and enlarge the range of resolutions over which it
has ``scaled-array capability'', (b) improve the dynamic range, field
of view and low-surface-brightness sensitivity of the VLBA, and (c)
provide the VLBA with a ``scaled-array'' capability similar to the
VLA's, which it presently lacks.
Figure: Possible antenna locations for a ten-antenna A+ configuration.
Eight new antennas are added in this configuration, at Dusty and
Bernardo
for the inner ring, and at Vaughn, Carrizozo, Truth Or Consequences,
Red Hill, Gallup and Bloomfield for the outer ring. This arrangement
results in much better imaging performance than the six-antenna
arrangement shown in Fig. 2.7.
New antennas would therefore be built in New Mexico and Arizona to
improve the density of coverage in the 35-400 km baseline range.
All proposed distributions include completing the ``Pie Town ring''
around the VLA by adding two new antennas at distances of 50 to 80 km
to the south and north-east of the VLA at Dusty, NM and Bernardo, NM.
Adding only the existing Pie Town VLBA antenna to the VLA (as in the
demonstration project that the NRAO is undertaking with NSF Major
Research Instrumentation funding) doubles the resolution of the A
configuration, but only for northern sources. Adding the two new
antennas at approximately the same distance from the VLA as Pie Town
extends this capability to the whole sky.
A further step is to add new antennas in the ``Los Alamos ring''.
Several configuration options are being investigated to determine the
cost-effectiveness of using different numbers of antennas in this
outer ring. Figure 2.7 shows the minimal arrangement
that was proposed in the . This places one new antenna in the
outer ring near Vaughn, New Mexico and a second antenna in this ring
near Holbrook, Arizona. This configuration fills the major holes in
the plane at most declinations, but denser coverage of this
baseline range, the configuration with eight new antennas shown in
Figure 2.8 would be needed to retain VLA-like image
quality with the A+ configuration resolution in the presence of
realistic noise, as demonstrated with example imaging below. The
configuration shown in Figure
2.8 approximates a 300-km ring of
seven equally-spaced antennas (to give uniform
coverage), including the Los Alamos
VLBA antenna, and centered on a line between Los Alamos and the VLA.
The antenna locations have been adjusted from the uniform spacing
on the ring to allow access from nearby major roads.
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.
A new software control system for the VLA will be required to interact
with hardware upgrades such as a new LO/IF transmission system and
correlator, to support new observing modes and strategies such as
mosaicing, dynamic scheduling and real-time imaging, to enable
more operator and user interaction with the data being produced by the
array, and to archive data at the expected maximum sustained data rate
of more than 10 MB/sec.
The VLA's on-line system is now based on two MODCOMP
computers of mid-1980's vintage, and the last major rewrite of the
control system occurred in 1989.
As part of the we propose to replace the software
and hardware controlling the VLA and the existing correlator with
modern technology. A design study is currently underway, including an
end-to-end analysis of information flow through the entire
proposal/observing/reduction process. We expect to
coordinate these efforts with parallel on-going VLBA and MMA projects.
Several other instrumental improvements that lead to new scientific
capabilities are also being considered. They include: a robust total
power system to support mosaicing, simultaneous multi-band performance
(2.4 and 8.4 GHz or 4.9 and 15 GHz), and outfitting the VLA antennas
with 80-GHz receivers.
Chapters 2-4 of the
described an ambitious scientific program that takes full advantage of
the project elements outlined above and imposes demanding performance
specifications on the enhanced VLA. In this section, we select a few
scientific highlights and mention their impact on the goals of the
. In making this selection, we emphasize projects for which the
changes the character of the work that can be done, or
introduces entirely new capabilities. Virtually all research programs
already active on the VLA will also benefit greatly from the proposed
improvements.
Solar radio bursts have long been observed using dynamic spectra
(records of intensity vs. time and frequency), and it is only
recently that joint spectroscopic and (fixed frequency) imaging
experiments have been conducted. An extremely exciting possibility is
imaging-spectroscopy of solar radio bursts over an octave, or more, of
bandwidth using a broad-band UHF system at the prime focus. Using such
techniques, it will be possible to constrain the point of origin and
the subsequent propagation of both electron beams (type III bursts)
and MHD shocks (type II bursts) in the solar corona. As such, it
offers a powerful tool for probing coronal dynamics, beam propagation,
and shock acceleration.
Unlike other proposals for feeds at the prime focus, a broad-band UHF
system does not need to be a high performance system. As such, it may
require the fewest modifications to the quadrupod and FRM. On the
other hand, it imposes demanding specifications on other aspects of
the project, requiring:
- A wide-band data transmission system and correlator.
- Dual circular polarization measurements.
- A spectral resolution of 1 MHz and a time resolution as
small as 10 ms, which may require the use of a ``burst mode''.
- Fast, accurate measurements of
.
Bistatic radar experiments with the VLA/Goldstone 8.4 GHz system are
an unanticipated use of the VLA which have produced a number of
exciting results, including: (i) the discovery of ice deposits at the
poles of Mercury, (ii) the discovery of the ``Stealth'' feature on
Mars, which produces no detectable echo, and (iii) the discovery that
Titan has no deep global ethane/ methane ocean, contrary to
expectations.
New 2.4 GHz receivers would enable bistatic radar experiments to be
done with both Goldstone and the newly upgraded Arecibo transmitter.
Furthermore, the VLA will be better able to support future planetary
missions if it is equipped for both the 2.4 and 8.4 GHz telemetry
bands.
To maximize support of bistatic radar experiments, the \
must include:
- Adding the 2.4 GHz
receivers.
- A correlator with a spectral resolution as high as 5 Hz.
- A correlator which supports simultaneous line and
continuum observing as well as simultaneous high- and low-resolution
line observing.
- Robust total power measurements.
At present, milli-arcsecond imaging is restricted to objects with high
surface brightness, nonthermal radio emitters. Large improvements
in
sensitivity coupled with the A+ configuration will allow imaging of
thermal radio sources with milli-arcsecond resolution, opening an
entirely new area of astrophysical inquiry. Examples of the kinds of
objects which could be imaged out to distances ranging from several
hundred pc to several kpc include i) interacting binaries containing
giant stars (symbiotic stars and recurrent novae); ii) stellar
winds on giant stars--expansion rates could be measured directly;
iii) the early stages of nova outbursts; iv) the photospheres of giant
stars, and v) circumstellar disks.
We stress that most elements of the VLA expansion project are
required simultaneously for these research programs, including:
- A wide-band data transmission system and correlator.
- The A+ configuration.
- Adding the 33 GHz band, completing the outfitting for 40 to 50 GHz
band.
- Correlator flexibility, including support of simultaneous line
and continuum observing.
Young stars are expected to be surrounded by protoplanetary disks. To
date, such stars have only been observed at optical wavelengths in
silhouette
against bright nebulae using, the Hubble Space Telescope. Their
inner regions are optically inaccessible since they are
opaque to visible photons.
The VLA 40-50 GHz system, when completed on all antennas and
supported by a wide-band data transmission system will have three
times the angular resolution of the Hubble Space Telescope and 36
times the sensitivity of the present 40-50 GHz system (a factor of
over a thousand improvement in observing time!). The VLA will be the
only
instrument able to penetrate the inner regions of protoplanetary
disks. The disks are optically thick to optical wavelengths, and they
cannot be resolved at longer wavelengths. Present estimates suggest
that roughly 100 protoplanetary disks with angular extent exceeding
exist within 200 pc of the Sun with total flux densities
of 1 mJy at 7mm wavelength. Such a source could be imaged with
a
signal-to-noise ratio of 10:1 in 12 hrs with the A-configuration.
Technical requirements essentially match those of §3.3.
The high sensitivity of the VLA at the high observing frequencies
(15 GHz and above) will open a new domain of rapid response to
transient phenomena that are initially optically thick at lower
frequencies. For example, to observe young extragalactic supernovae
in their earliest phases we need to be able to self-calibrate (or
phase-reference) sources fainter than 1 mJy in the presence of rapid
phase fluctuations in the wider VLA configurations. This is
impractical with the current VLA high-frequency systems because of
their relatively poor sensitivity and the need for long integration
times. The improved high-frequency coverage and wide bandwidths
planned for the enhanced VLA will allow good multi-frequency light
curves to be obtained for extragalactic supernovae even in their first
few days of activity. Rapid response and good data quality for this
phase of the outburst are crucial for testing particle acceleration
models, and for looking for inhomogeneities and/or instabilities in
the emission/absorption region. Similar considerations apply for
detection and early imaging of other transient phenomena, such as
X-ray transients and flare stars.
Technical requirements include:
- Multiple receivers covering 15 to 45 GHz.
- Wide-band IF transmission system for maximum sensitivity.
The will have a major impact on unbiased surveys in . Our
view of the large scale structure of the universe, with its large
filaments, sheets, walls and voids, is based almost entirely on
observations of high-luminosity galaxies, observed at optical
wavelengths. Since nearly all investigations of the properties and
spatial distribution of galaxies begin with optically (or IRAS)
selected galaxy catalogs, any population of gas clouds with very low
optical luminosity or surface brightness would largely have escaped
detection. Direct searches in the line circumvent these optical
selection biases. Such searches would make it possible to construct
an unbiased mass function for the local universe and to probe the
evolution of galaxies and the formation of large scale structure in
the range 0 < z < 0.8. Three examples of such surveys are:
- An All Sky survey in 0 < z < 0.2: the good surface
brightness sensitivity of the VLA D configuration, combined with the
large
instantaneous velocity coverage of the new correlator will finally make
such a survey feasible.
- The structure of a cluster of galaxies, combined with a pencil
beam
survey: at higher redshifts an entire cluster can be imaged within one
VLA primary beam, at z = 0.1, an Abell diameter is about
. In parallel, a sensitive pencil beam survey can be carried
out to sample the entire cone within this primary beam at 0 < z <
0.2 (RFI permitting).
- The evolution of gas at 0.2 < z < 0.8: a direct test of how
the gas content of galaxies evolves with redshift, using the
B configuration to survey a cone to as high a redshift as possible
(again, RFI permitting).
The large instantaneous velocity coverage of the new correlator will
make it possible for a single pointing and LO setting to cover the
range 0.2 < z < 0.8, and to sample a volume of
.
In a 100-hr integration more than thousand galaxies
will be detected with the upgraded VLA.
While Arecibo is more sensitive for directed studies, and the GBT
about as sensitive, the VLA's large field of view relative to
both single dishes makes it many times
faster than either for unbiased surveys. At higher redshifts (
0.06) the higher angular resolution and better interference
rejection of the VLA are also major advantages.
The technical requirements include:
- Robust total power measurements.
- Wide-band spectral line capability.
- RFI excision and extending the 1.4 GHz band down to
800(?) MHz, for the high-redshift survey.
The nature of the intragalactic medium (IGM) in clusters and groups of
galaxies is an important subject for understanding the large scale
properties of the universe. X-ray observations show this plasma,
which exists on a scale of hundreds of kiloparsecs up to at least many
Megaparsecs, has hot thermal gas as a major component. However, radio
observations show that, in many cases, large-scale magnetic fields
make up another important component of the medium. The physics of this
medium is important in its own right since it represents an extreme
environment to investigate complex plasma processes. How ubiquitous
and how well organized are the magnetic fields in the IGM? How were
they formed? Are dynamo processes responsible? What rôle do they
play in so called ``cooling flows''? Is the energy in the magnetic
fields, or that imparted to the medium by AGN's, important relative to
thermal content of the gas? Are relativistic particles accelerated
and/or re-accelerated on large scales? Are they energetically
important? What is the range of temperatures in the thermal IGM and
how is it regulated? The enhanced VLA will allow significant progress
on these issues, primarily through ultra-sensitive observations of i)
cluster radio halos; ii) Faraday rotation studies; iii) the
Sunyaev-Zeldovich effect; iv) radio galaxies in clusters; and v)
gravitational lensing and mass measurements of clusters.
Technical requirements:
- Broad-band data transmission system and correlator.
- Adding 2.4 and 33 GHz receivers.
- The E configuration.
The will open new ways to study the evolution of the radio
properties of galaxies with redshift directly. The strong (AGN)
radio source population clearly evolves with cosmological epoch -- the
apparent density of sources increases rapidly with redshift. This also
seems to be true for optically selected, often radio weak or silent,
QSOs. Not far below the 3CR flux density limit, most sources are at
high redshifts (z 
1 and many as high as z = 3 to 5). Recently,
the IRAS catalog has added a sample in which dust and molecular
emission can be detected beyond z = 2. Closer by, the Hubble Space
Telescope has imaged clusters of galaxies beyond z = 0.4, confirming
that the Butcher-Oemler effect (the blueing of cluster galaxy
populations with redshift) is related to increased star formation in
clusters at higher redshifts and that galaxies themselves have
different shapes as we look further back. The direct study of the
evolution and possibly the formation of galaxies appears to be a real
possibility. The enhanced VLA would play a crucial rôle in this
arena through detailed exploration of i) the evolution of radio source
populations; ii) molecules, dust and free-free emission in distant
galaxies; iii) starbursts at high redshifts; and iv) Faraday rotation
in magnetized gas-rich environments.
Technical requirements:
- Broad-band data transmission system and correlator.
- Multiple receivers covering 15-50 GHz.
- 2.4 GHz system.
The NRAO now operates 37 ``synthesis array elements'', broken into a
27-element sub-array (the VLA) and a 10-element sub-array (the VLBA),
with only one provision for cross-linking them (using one antenna or
the whole VLA in its phased-array mode for VLBI). Much could be
gained by allowing individual elements of the VLA, the
new antennas proposed for the A+ configuration, and the inner
elements of the VLBA, to join in observations with either array.
The information gathered by an N-element synthesis array
varies as 
for large N, so linking four new antennas to
the VLBA would
double the information content of VLBA observations by doubling the
number of correlations, and adding as many as eight new antennas
would more than triple the information collected.
This increased information content would
translate into improved image quality via
- a wider field of view,
- greatly improved sensitivity to extended structures,
- increased dynamic range and and image fidelity, especially for
larger emission
regions, and
- improved instantaneous, ``snapshot" image quality.
World-array experiments have shown that the apparent
simplicity of many VLBI images reflects a lack of information
gathered about the sources more than any intrinsic simplicity of
Nature on these scales. We expect many extragalactic sources to
have complex structures on scales from 1 to 100 milli-arcseconds.
Physical interpretation of these structures will require ``movies'' with
high dynamic range and wide fields of view over a large range in
frequency. For studies of rapidly-evolving galactic relativistic-jet
sources, we need the ability to construct such ``movies"
from snapshots of 
1 hour duration.
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.
The VLBA currently has limited
``scaled-array'' capability compared to that of the VLA.
Because of its small number of antennas, the
image quality deteriorates if any antennas are removed at one
frequency to ``match'' the coverage at another. It is also not
reconfigurable. Multi-frequency coverage at a fixed
angular resolution can be obtained only by adding data from baselines
shorter than 400 km at the higher frequencies (
Figure 4.2).
Cross-linking
an expanded VLA with the VLBA will extend the scaled-array capability
of both instruments to cover a much greater domain of angular
resolution, providing well-filled coverage that can be scaled with
wavelength
over a wide frequency range. For example, when studying relativistic
jets in AGN's or galactic X-ray transients, we must be able to image
total and polarized emissions at the same resolution over a wide range
of frequencies to determine spectral energy distributions, Faraday
rotations and magnetic field directions, as well as the collimation
and proper motion information that are available from single
frequencies.
Figure: VLBA images of the nucleus of the active galaxy
3C84 at five frequencies. Note the differing sensitivities. The
extent to which the detailed differences between these images are
really frequency-dependent properties of the source (rather than
differences in the coverage of the VLBA at these frequencies)
would be unclear even with matched sensitivities. The high-resolution
43-GHz image clearly under-samples the broader (>5 milli-arcsecond)
structure in the lower-frequency images, however and accounts for only
about 3/4 of the known total flux density. These uncertainties could
be much reduced by cross-linking the expanded VLA and the VLBA to
provide a scaled-array capability, especially at the highest
frequencies.
The VLBA ``snapshot'' coverage will be augmented to
produce better images of transient galactic sources that can vary
significantly in structure and amplitude on times scales of a few hours.
The VLBA configuration is fairly sparse and now relies on rotational
synthesis over several hours to obtain good image quality. The use
of the proposed A+ configuration antennas as VLBA
elements would significantly improve the ``instantaneous'' image
quality.
This can be critical to studies of the time development and
evolution of active galactic objects with relativistic jets, for
which high angular resolution imaging is appropriate but
``superluminal''
features may move by many beamwidths per day.
Although a separate ``100-km'' array (MERLIN) can provide some of
the ``missing coverage'' at >6cm for northern sources, the
high-frequency coverage that is critical to much of the astronomical
program, and the ``baseline synergy'' with the VLA or VLBA can be
obtained only with antennas in the Southwestern United States.
Furthermore, MERLIN has poorer image fidelity and sensitivity (having
only 4 to 7 antennas) and less frequency flexibility, than the
proposed .
Ultimately, to study how the properties of astronomical sources change
with frequency without changing resolution, we should be able to
select the antenna combinations needed to create the most appropriate
set of matched spatial filters for any given multi-frequency experiment.
We could do
this at high angular resolution if we were able to choose
``sub-arrays'' that include VLA and VLBA baselines almost
interchangeably. The NRAO prepared for this by siting the VLBA
antennas around the VLA as their centroid, and by co-locating the main
VLA and VLBA operations centers. The is needed to
exploit the unique opportunity that this creates. It gives us a way
to transform the scientific capabilities of two major
instruments significantly at once.
To take full advantage of this, we must outfit the new antennas and a
few existing VLA antennas with VLBA back-ends. The new antennas would
be operated as part of the VLBA when the VLA is in its more compact
configurations, and would need VLBA data acquisition systems. It is
also desirable to have enough hardware at the VLA site itself to
record signals from four VLA antennas at once. An upgrade of recording
and formatter systems to reach 1 or 2 Gbps is also desirable.
Astrometric VLBI observations would benefit from a 2.4 GHz/8.4 GHz
(S/X) dual frequency system. The VLBA correlator was designed to
handle 20-station experiments, so no new correlator capacity is
required specifically for the new antennas proposed as part of the
.
(under construction)
The addresses the demands of a wide variety of
scientific programs for greatly increased sensitivity, much broader
frequency coverage, enhanced spectral line capabilities, and better
angular resolution. It does so largely by returning the VLA to the
state-of-the-art in receiver technology, in the transmission and
processing of broad-band signals, and in correlator design. The
scientific requirements also pose new technological challenges. How
can optimum performance (polarization and sensitivity) be maintained
across the large bandwidths now proposed? Can broad-band,
high-performance, low-frequency feeds be designed? What is the
optimum way to transmit broad-band signals from antennas hundreds of
kilometers from the VLA for real-time ultra-high-resolution
interferometry?
The impact on astrophysics of returning the VLA to the current state of
the
art will be profound. Many hard limitations now
constraining VLA observations will be removed or greatly relaxed. The
continuum sensitivity will increase by ten-fold in several bands. New
frequency bands and increased bandwidth ratios will increase frequency
coverage almost three-fold. The bandwidth which can be processed by
the spectrometer, and its spectral resolution, will simultaneously
increase by about ten-fold. The minimum beam area will improve a
hundred-fold. Finally, cross-linking the upgraded VLA with the
VLBA will produce a VLBI instrument with greatly increased dynamic
range, field of view and frequency scalability relative to the present
VLBA.
The thus offers far more than an incremental
improvement to existing scientific capabilities. Almost all
areas of research now done with the VLA will benefit greatly from
it. It also provides fundamentally new science in many arenas.
Much of the new scientific capability depends on the cumulative effects
of
many improvements in the , rather than critically on any one of
them.
For
example, high-resolution imaging of stellar thermal emission requires
the sensitivity improvements and the A+ configuration; imaging
protoplanetary disks requires the 40-50 GHz upgrade and
enhanced sensitivity; deep surveys require extending the 1.4 GHz
band to lower frequencies and the new correlator.
If past experience is any
guide, this initial account of the possible scientific program, while
exciting, will prove to be far from complete. Some scientific arenas
that will benefit from the enhancement will almost surely have been
overlooked, and innovative uses of the improved instrument will not
have been anticipated.
We urge everyone in the VLA user community to add their
thoughts on the astrophysical goals and technical challenges of the
enhanced VLA to these discussions. This document is intended to
evolve into one that makes the scientific case to the NSF for
supporting these enhancements. All comments on it will be welcomed.
E-mail comments can be sent to
newvla@nrao.edu
This document was generated using the LaTeX2HTML translator Version 96.1-h (September 30, 1996) Copyright © 1993, 1994, 1995, 1996, Nikos Drakos, Computer Based Learning Unit, University of Leeds.
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The translation was initiated by Michael Rupen on Fri Mar 26 15:14:05 MST 1999
Michael Rupen
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