Issues and restrictions specific to VLA-EVLA
baselines
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A number of known problems is specific to crossed baselines, i.e.
baselines formed by a traditional VLA antenna and an upgraded EVLA
antenna. This class of problem does not occur on VLA - VLA or on EVLA
- EVLA baselines, and will automatically disappear once there are no
more VLA antennas left in the array. In other words, this is a
transition issue.
One way to avoid these problems is to flag all VLA - EVLA baselines,
but given the ever increasing number of EVLA antennas this means
throwing out a lot of useful data. Obviously these problems do not
affect observations which use only EVLA antennas only, such as when
observing outside the classical VLA bands.
Known problems of this type are:
- Whenever the (VLA) fluke synthesizers are reset there is
the potential for phase jumps on VLA - EVLA baselines. Such
resets occur a) on any frequency change, however small, and b) any
change in bandwidth. In either case, the observer should bracket
the source scan by calibrator scans at exactly the same
frequency/fluke and bandwidth settings
For this same reason, we do not recommend using Doppler tracking,
as the fluke changes resulting from Doppler tracking have the
potential to cause phase jumps. Further discussion of the use of
fixed-frequency observing, including the consequences for
dynamically scheduled projects, can be found in
this document
- The EVLA bandpass is enough different from that of the VLA that
about a 6% closure error is present on EVLA - VLA baselines in
continuum modes using 50 MHz bandwidths. This closure error will be
larger in continuum modes at narrower bandwidths. The closure error
can be mitigated by observing a strong source with known structure
and using the AIPS task BLCAL to determine baseline-based closure
corrections. For observers who are interested in continuum
observations for bandwidths of 25 MHz or narrower we recommend the
use of spectral line mode (i.e. pseudo-continuum), allowing the
effect to be dealt with with standard bandpass calibration
When observing in continuum, make sure to observe a baseline
calibrator at least once every hour in order to be able to run BLCAL
in the post-processing stage. This baseline calibrator should be a
source with known structure. By far the best is a strong (> 1 Jy)
unconfused point-source, such as a 'P' calibrator with no UV
restrictions for the frequency and configuration. A slightly
resolved source is acceptable, but not recommended, as an accurate
model of the structure is necessary to prevent erroneous solutions.
For details about determining and applying baseline solutions see
the relevant paragraph in
the post-processing section.
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Modified on Monday, 06-Oct-2008 14:49:47 MDT by Gustaaf
van Moorsel
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