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Previous revisions of this document can be accessed via the
EVLA Returns
Archive.
After the Modcomp computers were retired on Wednesday Jun 27, 2007 a
much more modern online system has taken over. While we have done all
we can to ensure that the most commonly used modes work under the new
system, we ask our users to inspect their data extra carefully, and
let us know of any unexpected effects as soon as possible
In January, 2008 it was discovered that small frequency differences
lead to phase changes, both for VLA and EVLA antennas. This makes it
impossible to transfer phase solutions from calibrator to source if
calibrator and source are observed at different frequencies. We
therefore strongly advise observers to observe source and calibrator
at identical frequencies, and not use Doppler tracking under
any circumstances until this problem has been fixed. A more extensive
description of the problem can be found here.
Two errors were discovered in how the new system calculates u,v, and w
coordinates. The data in the archive were corrected retroactively,
and no further correction during post-processing is needed. Observers
affected by this have been notified by e-mail.
For VLA antennas, when using an IF filter width below the default 50
MHz (25 MHz and 12.5 MHz) the new system does not guarantee that the
selected band fits in the width of the filter. We therefore strongly
recommend not to use values other than the default. Note that for
EVLA antennas such an IF filter does not exist, and specifying its
width has no effect on the EVLA signal path. A special case is
P-band, where a 25 MHz front-end filter is the default. All is well
as long as default P-band settings are used. If you use other
observing frequencies and/or bandwidths than the P-band default,
please contact us.
A number of problems or limitations related to having two different
systems controlling the array have now disappeared:
- When moving to the first source in an observation, VLA antennas
and EVLA antennas will now choose the same wrap
- For non-dynamic observations, durations are an acceptable
alternative again to stop times. Note that for dynamic observing,
durations continue to be mandatory
The following observing modes are now supported again:
- multiple subarrays. Note that:
- it is necessary to submit a separate observe file for each
subarray
- all subarrays must have the same correlator mode, bandwidth,
integration time, etc. In case of a conflict, the settings of the
subarray with the largest number of antennas will be used for all
others. For example, if the largest subarray has correlator mode
2AC and 6.25MHz bandwidth, all subarrays will use mode 2AC and
6.25MHz bandwidth.
- phased array
The following observing modes are not supported yet:
- planetary observations,
- solar observing
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