The following lists the main known problems in using the EVLA
antennas and their solutions/workarounds:
Known Problems (in red) and
Solutions/Workarounds (in black)
- In almost all modes, observing at 12.5
MHz bandwidth, spectral line mode, leads to a noise level 2 - 3
times higher than expected
Avoid using 12.5 MHz bandwidth
spectral line. More information can be found here.
- On EVLA - EVLA baselines power from
outside the band is aliased into the lowest 0.7 MHz of the band.
This effect is most serious at narrow bandwidths
We recommend not to use bandwidths
narrower than 0.78 MHz bandwidth. Particular care may be required
when observing with 0.78 MHz bandwidth. More information can be
found in the aliasing
- Resetting the (VLA) fluke
synthesizers is known to cause phase jumps on VLA-EVLA baselines
and are therefore a transition issue. Such resets occur at
changes in frequency and changes in bandwidth. See also the
special section on problems specific
to crossed baselines (VLA - EVLA).
On any frequency change, however
small, and at any change in bandwidth, bracket the source scan by
calibrator scans at exactly the same frequency (fluke settings).
Note that this also precludes use of Doppler tracking as the
resulting frequency shift will cause phase jumps.
- Closure errors of up to approximately
8% on EVLA-VLA baselines in 50 MHz continuum mode due to
non-matched bandpass shapes. The closure errors on narrower
continuum bandwidths are larger. See also the special section on
problems specific to crossed
baselines (VLA - EVLA).
Observe a strong source with known
structure (could be your amplitude calibrator) and use the AIPS
program BLCAL to determine baseline-based closure
corrections. Preliminary tests using this method reduced the
closure errors for the EVLA-VLA baselines by an least an order of
magnitude (in 50 MHz continuum mode). For narrower band continuum
observations, it is probably best to observe in spectral line
- EVLA antennas lack front-end Tsys
Recent versions of FILLM use
front-end Tsys values for VLA antennas and back-end Tsys values for
EVLA antennas. In older versions of FILLM specify CPARM(2)=2 to
use back-end Tsys values for all antennas.
- Use of IF filters (25 MHz, 12.5 MHz)
may lead to unexpected results for VLA antennas.
We advise to use the default IF
filter value (50MHz) only. For more info, see the new online system
- The last record of a scan is sometimes
erroneously labeled as the first record of the following scan.
This can have the detrimental effect that a record of data on a
calibrator is included with data on a target source.
We continue to suggest strongly
observers inspect their data for the occurrence of this problem,
and flag any affected records, e.g. using TVFLG. Alternatively,
one can use QUACK to flag the first record of every scan, whether
affected by this problem or not.
- Antennas sometimes fail to get
organized during the first scan.
For any observation, add a dummy
first scan. Typically, this would be a short scan on the first
source, and would be followed by the actual scan on that source.
Though the first 'dummy' scan could in principle be of arbitrarily
short duration, JObserve will insist on a non-zero dwell time for
- It is possible for automatic attenuation for IFs A/C
to be determined when the telescope is not yet 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, primarily at L-band, 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
This can be remedied 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. This effect is
strongest at L-band; as far as we know other bands do not require
such a dummy scan. For a much more extensive description of the
problem click here.
- In some bands, EVLA observations in IFs
A/C below the traditional lower VLA frequency limit suffer from
saturation issues caused by too large a frequency shift in the
signal path. IFs B/D will give useful data since their lack of
automatic attenuation prevents the saturation seen in IFs A/C.
We see this behavior in L-band below 1250 MHz and in C-band below
4400 MHz. For a more extensive description of the problem in the
L-band case click here
Below 1250 MHz (L-band) or 4400 MHz
(C-band), EVLA antenna data cannot be recovered for IFs A/C ---
only B/D data is usable
- Due to a clock error, data taken between January 4,
2008 at 05:00 UT and January 7, 2008 at 19:00 UT is corrupted as
follows: 1) a loss of ~25% in sensitivity on EVLA to VLA
baselines, and b) closure errors of up to 100% on EVLA to EVLA
baselines. VLA - VLA baselines are unaffected
On data taken in that time interval,
all EVLA - EVLA baselines need to be flagged. There is no
indication of closure errors on EVLA - VLA baselines, but
observers need to be aware of a 25% sensitivity loss.
- During the period June 27 to August 29,
2007 reference pointing solutions were not applied to antennas 1 -
In high-frequency observations
taken during this time period antennas 1 - 9 should be excluded
when deriving the flux calibration in GETJY