User Tools

Site Tools


lbaops:lbacalibrationnotes:since2010

Data correlated since 1 January 2010

All LBA data correlated at Curtin since 1 January 2010 do not have SEFD amplitude scaling applied at the correlator. This also coincides with providing output data in FITS-IDI format by default, rather than RPFITS as was previously the default.

For all data correlated since 1 January 2010, it is important to run AIPS task ACCOR as in some versions of DiFX used for production, the amplitude scaling for LBA data formats and/or zoom bands is incorrect. (Note that zoom bands must have been used for correlation of any observation with mixed bandwidth recording). Even if the scaling is OK, then ACCOR should still be run for 2 bit data to compensate for imperfect sampler statistics. This should be done as a first step before any other calibration.

For optimal weighting of the data in AIPS a priori amplitude calibration should normally be applied before fringe fitting.

For any particular LBA dataset, there are several possible strategies for amplitude calibration, depending on the level of accuracy required.

  • Nominal sensitivities can be assumed for all antennas. This may be acceptable e.g. for a faint source detection experiment, and may be the best one can do a priori. The nominal gain corrections can be applied using AIPS task CLCOR: run once for each antenna with OPCODE='GAIN' and CLCORPRM(1) set to the square root of the nominal SEFD.

or

  • Measured Tsys and gain curves for each antenna can be applied where available, using AIPS tasks ANTAB and APCAL. More detailed instructions can be found here. The measured Tsys values should be inspected as sometimes there are problems with the logs (e.g. if amplitude variations are made worse by applying the “corrections”, you should worry). The availability and quality of measured Tsys is generally poor, especially at the present time as new backends are being employed at many telescopes and currently have limited ability for Tsys monitoring.

or

  • Spectral line users often calibrate their data using total-power spectra in AIPS task ACFIT, to generate an SN table which contains the relative gains of the antennas as a function of time.
  • After fringe-fitting and applying nominal or measured Tsys/gain corrections, some improvement may be possible via imaging (or model-fitting) and self-calibration on calibration sources or bright target sources. These gain (both amplitude and phase) corrections can then be transferred from a calibrator to a nearby faint target source. Gain amplitude solutions from self-calibration should be carefully inspected, as it is possible to introduce large errors in the case of a small number of antennas and sparse (u,v) coverage - which is often the case for LBA data.
  • If the VLBI structure of the calibration source(s) is well known, and/or VLBI or total flux density monitoring data exists, and/or the simultaneous local ATCA data are available, then these additional data can provide useful checks to estimate the flux density on LBA baselines and gauge the accuracy of the VLBI calibration/adjust scaling factors where necessary. VLBI characteristics of a calibrator can be searched in the Radio Fundamental Catalog compiled by Leonid Petrov, and ATCA characteristics in the ATCA calibrator database. Very few sources are completely unresolved both on short and long baselines, i.e. so that their flux density is the same on both ATCA and VLBI baselines. However, some sources are very compact and have only a tiny fraction of flux density “missing” on intermediate scales between ~6 km and ~100-1000 km baselines. (Note that such sources are often highly variable!) In that case the flux density measured simultaneously from the local ATCA data, which can usually be determined to a few percent accuracy relative to the primary calibrator PKS 1934-638, can help to provide a good estimate of the LBA flux density. (e.g. PKS 1519-273 is one source that is highly variable, and shows interstellar scintillation on intra-day timescales, but is effectively unresolved on baselines up to ~1000 km. So it can be used to “bootstrap” instantaneous flux density between the ATCA and VLBI datasets, i.e. to adjust the scaling factors of the LBA antennas to obtain more accurate flux density estimates, since ATCA and the LBA see effectively the same flux density.) However note that simultaneous flux density measurements are not generally possible with CABB VLBI modes (except the hybrid 64 MHz/1MHz mode).
lbaops/lbacalibrationnotes/since2010.txt · Last modified: 2015/12/18 16:38 (external edit)