Configurations Overview & VLBI
Opportunities
Bryan Butler – NRAO
U.S. Radio/Millimeter/Submillimeter Science Futures II
U.S. RMS Science Futures II
AUG Aug
3, 2016 3, 2016
ngVLA Configurations
One of the most important design decisions for a
synthesis array is the configuration of the antennas.
Along with sheer number of antennas, it determines or
affects:
– Resolution
– Imaging quality
– Brightness temperature sensitivity
– Calibration strategies
And there are many cost and operational implications
which are associated with decisions on antenna
configuration(s).
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ngVLA Configurations
The Good:
– Whatever size and number of antennas we arrive at in
the end for ngVLA, the number will be large (of order
hundreds), which allows quite a bit of flexibility.
– There has been a tremendous amount of time and
effort put to this problem in the past (e.g., 39 memos
in MMA/ALMA memo series alone – maybe as many
for SKA).
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ngVLA Configurations
It’s fairly straightforward to
make a toy configuration
which has Gaussian
distribution of baselines,
good u-v coverage, and
other desirable properties
(c.f. Boone 2001, 2002;
Conway 1998, 2000).
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ngVLA Configurations
Or other
configurations
which have similar
desirable
properties (Clark
& Brisken 2015;
based on Conway
2000).
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ngVLA Configurations
The Bad:
– Despite how much work has gone into, for example,
optimizing antenna configurations for imaging, that
optimization depends strongly on the science (notably
the size and spatial structure of sources). For a
general purpose instrument, and especially one that
may not be reconfigurable, difficult decisions (and
compromises) will have to be made.
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ngVLA Configurations
The Ugly:
– Even if we could come up with an acceptable
“optimal” configuration, the practicalities of where we
can sensibly place antennas (due to availability,
accessibility, power, fiber, etc.) will almost certainly
outweigh the desire to have that ideal configuration, at
least for the longer baselines. This is because the scale
which we’re currently considering (hundreds of km) is
larger than local geologic/geographic scales.
AUG 3, 2016
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ngVLA Configurations
Because of these practical considerations, and the underlying
science, it is instructive to think of the ngVLA as consisting of
three parts:
– A short baseline component, with a large number of
antennas out to 1’s of km (co-located with current VLA).
– An intermediate baseline component, with antennas
distributed on the plains of San Agustin, out to 10’s of km.
– A long baseline component, with antennas out to 100’s of
km.
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ngVLA Configurations
Short Baseline Component
• Main driver is sensitivity to large-scale structures (high brightness
temperature sensitivity).
• Many antennas out to few km diameter. Say 1 km maximum
baseline, with 18 m antennas, then 10% filling factor would be of
order 100 antennas.
• Maybe some fraction of
these in an even more
compact core (20
antennas in 200 m is
almost 50% filled)?
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ngVLA Configurations
Short Baseline Component
There is still a hole in the u-v plane on the scale of the shortest
spacing between antennas – how to fill it?
• Some antennas total power?
• Array of smaller antennas?
• Larger antenna?
• Some combination?
Does it even need to be filled for the
most important science?
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ngVLA Configurations
Intermediate Baseline Component
• Bridges the gap between the short and long baselines; needed for
good imaging on intermediate scales.
• Ideally want “smooth” distribution of baselines connecting to
short baseline component (some might argue Gaussian, some
might argue power law).
• Antennas distributed on the plains of San Agustin, out to 10’s of
km.
• Land access already becoming an issue, at this scale.
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ngVLA Configurations
Intermediate Baseline Component
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ngVLA Configurations
Intermediate Baseline Component
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Long Baseline Component
• Need long baselines for resolution, of course. But cannot only
have a few antennas or imaging will suffer.
• Ideally want “smooth” distribution of baselines connecting to
intermediate baseline component (some might argue Gaussian,
some might argue power law).
• Antennas distributed across state of NM (AZ and TX also?) and
maybe into Mexico, out to 100’s of km. For scale, note that the
state of NM is of order 550 km across.
• Land availability, access (topography, roads), power, fiber now a
serious issue.
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ngVLA Configurations
Long Baseline Component
Easy to plop down a 300
km diameter circle on a
map, but difficult to find
good locations for
antennas.
Topography is an issue.
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Long Baseline Component
As is land availability and
access (roads).
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Long Baseline Component
And power.
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Long Baseline Component
And access to fiber.
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Long Baseline Component
Frazer Owen and Eric
Greisen have started to
address some of this. But
many uncertainties remain.
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ngVLA Configurations
Longer Baseline Component
• We have components on 1’s, 10’s, and 100’s of km – the next step (1000’s
of km) is the territory of VLBI.
• In fact, the shortest baseline of the VLBA (LA-PT) is 236 km; shorter than
the few hundred km maximum baseline we’re discussing for ngVLA.
• We should retain the capability of using the longest baselines of ngVLA
along with the VLBA, and even adding some ngVLA antennas (or stations)
on somewhat longer baselines (many hundreds of km, at least).
• A more ambitious possibility – absorb the VLBA antennas into ngVLA. The
VLBA antennas (or replacements thereof by ngVLA antennas or stations)
then simply become the longest baseline component of ngVLA.
• Software becomes more complex (scheduling, observing techniques,
correlation, data reduction, etc.), but the scientific payoff can be significant,
and the operations gain should not be ignored.
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ngVLA Configurations
Implications for Phase Calibration
Techniques of phase calibration may have implications on
configurations of antennas:
• The paired-antenna or calibration/reference array technique of
phase calibration requires multiple antennas (though not
necessarily all identical) per location.
• This might be appropriate for the long baseline component, but
perhaps not for the intermediate, and probably not for the short.
• See Dave Woody’s talk.
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ngVLA Configurations
Reconfigurability
• The possibility of moving antennas, at least in the short baseline
component and possibly even in the intermediate (but probably
not in the long) is intriguing. Reconfigurability certainly allows
more flexibility to a broader range of science.
• The difficulty is cost – for roads, transporters, and antennas.
• See Jim Condon’s talk.
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ngVLA Configurations
Open Questions
• What is the size and number of antennas?
• What is the longest baseline?
• How much relative collecting area in the main three components
(short, intermediate, and long baselines)?
• Do we need to fill the u-v hole, and if so how?
• What are the implications for phase calibration (or vice versa)?
• Should it be possible to reconfigure the short and/or intermediate
baseline components?
• For the long baseline component, is it possible to get reasonable
antenna locations given issues of land availability, access, power, fiber,
etc.?
• How much integration with VLBA should be pursued?
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