Known Problems and Solutions

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)

  1. 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.

  2. 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 section.

  3. 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.

  4. 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 (pseudo-continuum) mode.

  5. EVLA antennas lack front-end Tsys values

    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.

  6. 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 section.

  7. 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.

  8. 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 that scan

  9. 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 affected.

    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.

  10. 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

  11. 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.

  12. During the period June 27 to August 29, 2007 reference pointing solutions were not applied to antennas 1 - 9

    In high-frequency observations taken during this time period antennas 1 - 9 should be excluded when deriving the flux calibration in GETJY

Modified on Tuesday, 29-Jan-2013 13:57:48 MST by Gustaaf van Moorsel