Determining Gains and Polarimetric Properties - GPCAL

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 primary 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:

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 primary, 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.

2016-06-21