Observing Strategy
If you have L-band time scheduled, you will need to obtain the latest
version of OBSERVE
or JOBSERVE
(a GUI-based Java-version of the OBSERVE program). When
submitting your observe file to the VLA operators (observe@nrao.edu, email as body of
the text) it is recommended to route a copy to the Data Analysts (analysts@nrao.edu) who will look
at your file and, if necessary, get back to you with advice. Consider
the following guidelines before submitting your schedule.
General Considerations
Active Sun poses a serious problem for sensitive L-band observations
in any of the VLA configurations. Solar Flares with as much as a
million Jy at L-band and narrow angular extents are a source of
interference. These flares are equivalent to bright unresolved sources
with time-varying flux densities making it very difficult, if not
impossible, to remove their effects. As a consequence, the resulting
images will be of poor quality and low dynamic range. The Solar
activity is a function of the Solar Cycle. Quiet Sun also poses
problems for L-band observations, particularly in the short
configurations (D - 1 km, C - 3 km). In the larger configurations
like the B (11 km) and the A (36 km) arrays one can expect the effects
of the Quiet Sun to be reduced. Sensitive L-band observations are best
carried out during the night.
Continuum Observations
The default continuum observing mode with a 50 MHz bandwidth is useful
if one is only interested in imaging a small portion of the primary
beam and is not expecting to obtain noise-limited, or,
confusion-limited images. The limitations are due to the inability to
identify and remove interference which is averaged over the bandwidth,
and the limited field of view imposed by the bandwidth decorrelation
due to the larger bandwidth. In addition, the bandwidth decorrelation
makes it harder to identify and remove the confusing sources away from
the phase center since they are radially smeared, thereby limiting the
dynamic range of the images. Wide-field continuum images can only be
produced by observing in the multichannel continuum mode.
Choice of frequency and bandwidth
Take a look at the recommended center frequency and bandwidth
combinations for L-band observations given in
Table 11. If one is intending to use 50 MHz bandwidth it is not
possible to use multichannel continuum mode with 4 IFs since this has
inherent problems. It is possible to use the 2 IF mode with 8 channels
per IF. The channel width in this mode is not narrow enough to avoid
the problems mentioned earlier. The recommended bandwidth is 25 MHz
with 4 IFs and 8 channels per IF. This is Correlator Mode 4. This
combination gives narrower channels resulting in improved interference
detection and removal, in addition to being able to image a wider
field of view. The two recommended center frequencies are 1365, and
1435 MHz. Alternative choices of center frequencies are possible in
consultation with the latest
interference plot and
Table 11 . Note the OBSERVE and JObserve names of L-band standard
frequencies in
Table 12 .
Choice of Integration time
Time-averaging of visibilities results in tangential smearing and
consequent amplitude loss for sources not located at the phase
center. The magnitude of the amplitude loss depends on the distance of
the source from the phase center and the antenna configuration. This
amplitude loss manifests itself in the images as a tangential smearing
of the sources. The following
Table 9 gives the maximum integration time for a given
configuration if the amplitude loss for a point source at the first
null of the primary beam should not exceed the value in the table.
Quite often deep imaging of the desired field of view requires imaging
beyond the first null and up to the first sidelobe. Under such
circumstances the averaging time needs to be correspondingly shorter.
The recommended integration time in the A-configuration is 3.3
sec. This choice of integration time keeps it synchronous with the
period (1.66 sec) of the Walsh Switching and minimises
systematics. The integration time can be correspondingly longer (in
integral multiples of 1.66 sec) in the shorter configurations. The
recommended integration times in the B, C, and D configurations are 5,
10, and 30 secs respectively. However, read the notes on Minimum and
Maximum available integration times. A consequence of
time-averaging loss is smearing of point source responses far away
from the phase center that makes them appear extended. At the moment
there is no standard algorithm which can 'deconvolve' this effect and
estimate the true sizes of sources. The current best solution to this
problem is to minimize the averaging time such that the amplitude loss
and hence the smearing is insignificant.
Choice of Hour Angles
The L-band system temperatures of the VLA antennas increase with the
decreasing elevations of the antennas. The lowest system temperature
is at zenith and is about 35 K. While the increase in the system
temperature with decreasing elevation is different for different
antennas, on the avearge it is about a factor of 2 at elevations of 15
degrees compared to the zenith values. Consequently, low-elevation
observations will have a weighting which is lower by the square of
this factor. Since the sensitivity of the VLA deteriorates at low
elevations, it is recommended that L-band observations be carried out
at the highest elevations that are practical. These elevation
restrictions will have a corresponding range of hour angles depending
on the declinations of sources of interest. A desirable choice of
hour angles is one in which the system temperature does not increase
by more than a factor of 1.5 compared to the values at transit.
Calibration
It is recommended that the primary flux calibrator be observed at
least once in an observing run. The secondary flux calibrators, which
are used to estimate the overall antenna gains, are observed more
often, about once in an hour. For multichannel continuum observations
the primary flux calibrator can also be used as a bandpass calibrator
to correct for the relative gains of the spectral channels.
Example Observe File
A typical observe file for a multi-channel continuum imaging in the A
array is given
here. Note that the observing mode is 'line'. In this example, 4
IFs are recorded (code DS 4) each with a baseband bandwidth of 25 MHz
(code 1111). The number of channels available in this option are
8. The channel width is 3.125 MHz. Although this is a reasonable
choice of channel width for wide-field imaging, bandwidth
decorrelation effects on sources even within the primary beam can be
seen in A-array images. Take a look at this
image. This source is about 22 arc min from the phase center (and
pointing center). The radial smearing of the source due to bandwidth
decorrelation is evident. In addition, there are some residual
sidelobes although this is a CLEANed image. These alternating positive
and negative sidelobes are also expected as a consequence of bandwidth
smearing. Care should be exercised in attributing sizes to sources
away from the field center, since even unresolved sources will appear
extended as shown in this example. The effects of bandwidth
decorrelation are minimized when the product of the ratio of the
channel width to the center frequency, and the ratio of the separation
of the source in synthesized beamwidths is less than 1. In the current
example, this product is about 2. The integration time used in the
above observe file is 3.3 sec. Note that the secondary calibrator
(1035+564), which also serves the purpose of a bandpass calibrator, is
observed about once in an hour for about 2 minutes. The primary flux
calibrator (1331+305) is observed towards the end for about 10
minutes. The source 1046+59 is observed for about 30 minutes in each
scan. Scan durations longer than 60 minutes are not preferable. The
'//AN' line is an 'Antenna Card' activated here to make sure that the
Antennas use the correct azimuth loop and do not waste time wrapping
and unwrapping the azimuth cables. This is an option that can be set
while creating the observe file.
Spectral Line Observations
Please consult the Spectral
Line Users Guide.
Modified on Friday, 26-Sep-2008 12:09:11 MDT by Gustaaf van Moorsel
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