Task: gpcal Purpose: Gain/phase/polarization calibration of dual feed data. Categories: calibration Gpcal is a MIRIAD task that determines calibration corrections (both antenna gains and instrumental polarisation characteristics) for an array with dual feeds, from an observation of a point source. The source can be polarised, with unknown polarisation characteristics. Though the source may be strongly polarized, the instrumental polarisation errors are assumed to be small (of order at most a few percent). Normally GPCAL writes the solutions as a gains table (item 'gains') and a polarization leakage table (item 'leakage'). With nfbin>1, GPCAL writes additional tables 'gainsf' and 'leakagef' with solutions for frequency binned data. Not many other tasks currently read these tables, but they will be applied in preference to the single frequency versions if present. GPCAL can handle either dual linear or dual circular feeds. However by default it expects dual linears -- you must use options=circular so switch it to circular mode. Also the terminology in this document is aimed at linears (e.g. we talk of "XY phase" -- for dual circulars this is really "RL phase"). Note that the user specifies which parameters are to be solved for. In the case of leakages and xyphases, GPCAL will check for the existence of items "leakage" and "gains" in the input data- set. If present, these will be used as the initial estimates of these parameters. If you are not solving for these parameters, they will be held at their initial value through the solution process. After converging on a solution, and if the xyphase offsets or leakage parameters have been modified, GPCAL will write out their current values to the appropriate items. Conventions: Unfortunately there has been a number of changes in the "sign conventions" used within Miriad. For a discussion of the conventions, past sign errors and how they affect you, see the memo "The Sign of Stokes-V, etc." by Bob Sault. Key: vis Input visibility data file. The data should be either raw linear or raw circular polarisations. No default. The visibility data must be in time order. Key: select Standard uv selection. Default is all data. Key: line Standard line-type specification. Multiple channels can be given. Generally it is better to give multiple channels, rather than averaging them into a "channel-0". The default is all the channel data (or all the wide data, if there is no channel data). Key: flux The values of the I,Q,U,V Stokes parameters. If no values are given, and it is a source known to GPCAL, GPCAL uses its known flux as the default. If GPCAL does not know the source, the flux is determined by assuming that the rms gain amplitude is 1. If the option "qusolve" is used, the given fluxes for Q and U are used as the initial estimates. Also see the oldflux option. You may specify an I,Q,U,V flux density for each of the nfbins that you request, but all values must be present. If you set nfbins higher than 1, then any bins without information provided here will use the values from the first bin. If you want to use spec to describe the Stokes I flux density, then you only need to put the flux density at the reference frequency as the first parameter here, while all other I values are ignored. Key: spec The reference frequency (GHz), spectral index and up to two higher order terms. Only used if nfbin>1. The spectral index terms default to zero. Key: refant The reference antenna. Default is 3. The reference antenna must be present throughout the observation. Any solution intervals where the reference antenna is missing are discarded. Key: minants The minimum number of antenna that must be present before a solution is attempted. Default is 2. Key: interval This gives one or two numbers, both given in minutes, both being used to determine the extents of an amplitude calibration solution interval. The first gives the max length of a solution interval. The second gives the max gap size in a solution interval. A new solution interval is started when either the max time length is exceeded, or a gap larger than the max gap is encountered. The default is max length is 5 minutes, and the max gap size is the same as the max length. The polarisation characteristics are assumed to be constant over the observation. Key: nfbin The number of frequency bins. The default is 1. Use nfbin>1 to solve for variation across the band in the gain and leakage parameters. Works best for uv files with a single spectral window, i.e., after uvsplit. Key: tol Error tolerance. The default is 0.001 which should be adequate. Key: xyphase Generally the use of this parameter has been superceded. Initial estimate of the XY phase of each antenna. The default is 0 for all antennas. If the XY phase has not been applied to the data, then it is important that this parameter is set correctly, particularly for the reference antenna. Key: options These options determine what GPCAL solves for. There are many permutations, the more obscure or useless of which are not supported. The option values are used to turn on or off some of the solvers. Several options can be given, separated by commas. Minimum match is used. Some combinations of these options are not supported. xyvary The XY phase varies with time. By default the XY phase is assumed to remain constant. qusolve Solve for Q and U fluxes. Good parallactic angle coverage is required for this. oldflux This causes GPCAL to use the pre-August 1994 ATCA flux density scale below 11 GHz and the pre-May 2016 mm fluxscale above 11 GHz. See the help on oldflux for more information. nopol Do not solve for the instrumental polarisation leakage characteristics. The default is to solve for the polarisation leakages on all feeds except the X feed of the reference antenna. noxy Do not solve for any XY phase offset. The default is to solve for the XY phase offset on all antennas except for the reference antenna. nopass Do not apply bandpass correction. The default is to apply bandpass correction if possible. This is rarely useful. Use with caution. noamphase Do not solve for the amplitude and phase. The default is to solve for amplitude and phase. This option is rarely useful. linear Expect/handle data from feeds that are linearly polarised. This is the default. circular Expect/handle data from feeds that are circularly polarised. The following are options for advanced users, and should be used with forethought and caution. reset If calibration tables (leakage and gains) are present within the input dataset, GPCAL usually uses these in determining default XY phases, leakages and flux scale. This is usually the desired behaviour. The "reset" option suppresses this behaviour, and starts GPCAL with a clean slate. xyref Solve for the XY phase of the reference antenna. To do this, the source must be linearly polarized and you must have reasonable parallactic angle coverage. This option can be used with "noxy", in which case GPCAL solves for the offset of the reference antenna. polref Solve for the instrumental polarization leakage of the X feed on the reference antenna. This can be combined with "nopol", in which case GPCAL solves for X feed of the reference antenna only. vsolve Solve for the Stokes-V of the source. This is only possible for linear feeds and a preliminary leakage solution for the observation already exists. This preliminary solution must be formed from a calibrator with known Stokes-V. Revision: 1.23, 2020/11/23 04:42:34 UTC

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