Issues and restrictions specific to EVLA antennas
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Observers should inspect data from the EVLA antennas particularly
carefully, since not all the online system flags are currently
operating for these antennas. At present, flags are set for antennas
that are off source, mispositioned subreflectors, and synthesizers
that are out of lock.
This section lists a number of issues which are particular to the
current EVLA antennas, and therefore to any baseline involving at
least one EVLA antenna. We expect to resolve most of these issues
over time, and we will update this list accordingly.
- It is possible for automatic attenuation for IFs A/C to be
determined before the EVLA antenna is on source, causing
incorrect values for the flux density. For IFs B/C there is no
such automatic correction at all. This only affects very strong
sources, capable of increasing the antenna temperature by a factor
~2. Given a 1 degree increase for every 10 Jy it follows that
sources stronger than around 500 Jy are affected. At L-band
continuum, these are Cas A, Cyg A, Tau A, and Sag A. For line
observations, only very strong masers may be affected. This can
be remedied for IFs A/C by observing a dummy scan on source before
any scan on source, allowing the correct attenuation to come into
effect during the dummy scan. Similarly, insert a dummy scan on
the source following the strong source in order to reset the
attenuation. For IFs B/D the attenuation settings are derived
from a table and are therefore wrong in the case of strong
sources, leading to unusable data. For a more extensive
description of the problem click here.
- EVLA L-band observations below 1250 MHz (currently only possible
with antenna 14, but the number is expected to grow) in IFs A/C
suffer from saturation issues caused by too large a frequency
shift in the signal path. This is a transition issue and will go
away once the WIDAR correlator becomes operational. IFs B/D will
give useful data since their lack of automatic attenuation
prevents the saturation seen in IFs A/C. For a more extensive
description of the problem click here.
- Use of the round-trip-line-length monitor data is not
implemented. The net effect is a diurnal term in the antenna
phase. This should not be a problem (except for astrometry) with
reasonably frequent calibration.
- Observers need to examine the baselines including EVLA antennas
more carefully than is needed for unmodified VLA antennas, and
may need to flag their data more extensively. In particular the
first and last integration of a scan should be scrutinized
- Since online flagging for EVLA antennas is not as mature as it is
for VLA antennas, the online system may flag good data on
baselines involving an EVLA antenna. Those concerned about
losing data for this reason are advised to fill the data a second
time, using the CPARM(2) = 1 and CPARM(3) = 16 in FILLM, thus
allowing online flags to be ignored. To get both the backend
Tsys and flags off, set CPARM(2) = 3 and CPARM(3) = 16.
- EVLA antennas lack Tsys values. In order to obtain a Tsys value
for EVLA antennas use the VLA back-end Tsys, which can be
obtained by setting CPARM(2) = 2 in FILLM.
- The EVLA L band (20 cm) and C band (6 cm) feeds are new. Their
properties are not yet well determined. In particular, the
variation of polarization across the antenna beam has not been
measured.
- The EVLA high-frequency performance is not as good as that of VLA
antennas, due to a combination of issues that have not been
completely resolved which includes pointing, subreflector
positioning, and focus. A few issues pertaining to high
frequency observations:
- Tipping curves for opacity determination are not implemented on
the EVLA antennas. In most cases this will not affect users, who
at this point should be using the opacity corrections derived
from the weather data that are calculated by FILLM in AIPS when
running with the default BPARM=0.
- Subreflector focus and rotation with elevation is not yet
implemented, nor is the special rotation correction at K and Q
band. Thus the gain response versus elevation is likely to be
significant, and to change on relatively short timescales for
these antennas until the retrofits are complete. These effects
can to some extent be taken into account by observing your
primary flux calibrator at an elevation as close as possible to
that of your phase calibrator, so that its flux can be
bootstrapped while minimizing systematic elevation-dependent
offsets. The routine ELINT in AIPS can also help to alleviate
elevation-dependent gain offsets between your source and phase
calibrator.
- Secondary reference pointing was implemented and tested on EVLA
antennas in mid-July 2006. Observers should pay careful
attention to their secondary reference pointing results on EVLA
antennas, and report any apparent problems.
- The High Time Resolution Processor is not available with the EVLA
antennas
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Modified on Tuesday, 29-Jan-2013 13:57:48 MST by Gustaaf
van Moorsel
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