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Determining Gains and Polarimetric Properties - GPCAL

Task gpcal is the main workhorse of the Miriad calibration system, although it is only particularly useful if you have measured all four polarisation products (XX, YY, XY and YX). It has a plethora of options to turn on and off various solvers. Most of these will be irrelevant to normal use - particularly when calibrating a source, such as 1934-638, which is known to be unpolarised. However it does not determine a bandpass function. So, if you have not averaged your data into a channel-0 dataset, you should precede gpcal with mfcal, or if you are using a separate bandpass calibrator, ensure that its bandpass solution is copied across using gpcopy - gpcal will normally apply any bandpass function it finds with the dataset before it performs real work.

Do not be intimidated by the number of options - we give you advice on which ones to use in the following chapter. We now discuss the various inputs.

• vis: The input dataset.
• line can be used to select the range of channels, and the averaging to be performed on a multi-channel dataset. See Section 5.4 for more information. Generally you should not specify any averaging. Task gpcal performs its own averaging in a fashion which gives best results.
• interval: This interval parameter has the same meaning as that described in Section 11.4 above, and the advice for setting it is the same.
• flux gives the values of the four Stokes parameters for your calibrator source. If you choose to solve for Q and U, (see options below) then these are initial guesses only - it is not necessary to give them accurately, or at all, in this case. Make sure you get the signs of Q, U and V right; these are real numbers, not visibility amplitudes. Note that gpcal does recognise a small group of calibrators for which it knows the spectrum in I, and sometimes Q and U - see the help file for details. The flux calibrator 1934-638 is amongst these. If gpcal does not recognise the source, the default is an unpolarised source. This will be quite inappropriate if you have a polarised source and do not solve for Q and U. If no flux density is given, and the source is not known, then gpcal assumes that the rms gain amplitudes are 1 - and determines the calibrator flux density accordingly. This will be a good approximation if you equalised the gains at the start of the observation (which is the normal practise). However, at this stage, this approximation is largely a convenience - any error in the assumption will be corrected later.

• tol: The iterative procedure converges when the solutions are unchanged from the previous solutions by an amount tol. The default is 0.001. Make this smaller if you want more iterations.

• xyphase: This parameter is an artifact of history - it can be ignored when following the current recommended procedure for calibrating ATCA data. This parameter can be used as an alternative way of specifying the XY phase of the antennas. If the XY phase on the reference antenna is constant to good approximation, and provided you have not already applied any XY phase correction (i.e. neither in AIPS or Miriad atlod nor with atxy), then you can give the XY phases of the antennas here. Unless you explicitly turn off solving for XY phase in gpcal, then the only important XY phase value is that for the reference antenna. All the same you should give values for all antennas. One significant catch (for arcane reasons) is that the XY phase reported by AIPS is not the XY phase used by Miriad. They are related by:
  
  $$\phi_{miriad} = \left\{ \begin{array}{ll} \phi_{aips} - 180... ...80 & \mbox{if sideband indicator is -1}\\ \end{array}\right.$$
  
  
  The value of the sideband indicator is noted in the history generated by ATLOD. More simply, however, it will also be the sign of the frequency increment.

• options controls what gpcal solves for. Several options, separated by commas, can be given. It is important that you understand the different choices.

  oldflux

  If you are calibrated data that are to be combined or compared with ATCA data reduced before August 1994, you will generally want to use the `oldflux' option to select the pre-August 1994 ATCA flux scale. See Section 12.6 for more information.

  qusolve

  means that gpcal will solve for Q and U, taking the model you gave in flux as a starting point. You need many cuts of the calibrator with good parallactic angle coverage to do this successfully. You should not attempt to solve for Q and U of the flux calibrator, as you will invariably have too little data. Additionally 1934-638 is known to be unpolarised.

  xyvary:

  If the telescope settings are not altered, the XY phases (which are purely instrumental) appear to remain constant to better than a few degrees. By default, gpcal assumes the XY phases are constant. Because of the reliability of the XY phase measurement system, this assumption is now known to be inappropriate. It is better to let gpcal solve for the XY phases as a function of time - use option xyvary for this. Note that gpcal can do this for all antennas except the reference antenna - the reference antenna is assumed to be constant and (generally) zero.

  xyref

  means that gpcal will solve for the XY phase of the reference antenna. To do this, the source must be strongly polarised.

  noxy

  means that gpcal will not solve for any XY phases. By default it solves for all XY phases, except for the reference antenna. This is generally not an appropriate switch to use.

  polref

  means that gpcal solves for all the leakage parameters of the reference antenna. By default it does not attempt to solve for the misalignment and ellipticity of the X feed of the reference antenna. It is only possible to use this option if the source is strongly polarised (at least 5%). If you specify Q and U, both these terms can be found. However, if you ask for the qusolve option, then the misalignment term cannot be determined.

  nopol

  means that gpcal does not solve for the polarisation leakage terms. You must use this option if you are calibrating data without XY or YX correlations.

  noamphase

  means that gpcal does not solve for the antenna gains. This option is rarely useful.