Issues and restrictions specific to EVLA antennas

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

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