Documentation on varfit

Task: varfit
Purpose: Analyse correlations of antenna gains and uv-variables.
Categories: uv analysis

VARFIT is a Miriad task to analyse the correlations of
        antenna gains and uv-variables. VARFIT fits the following:
        	 yaxis = slope *  xaxis +  intercept
        	D(yaxis) = slope * D(xaxis) + intercept
        where D() is the difference between succesive samples.
        If either axis is an antenna gain, then SELFCAL must be run first
        and the axes are sampled at the interval used in SELFCAL.
        The rms and correlation coefficient are listed for each antenna. 

Key: vis
        The input UV dataset name. The antenna gains must first be
        derived for the selected linetype by the task SELFCAL with
        the desired averaging interval.

Key: xaxis
        xaxis can be antenna gain ``amplitude'' or
        ``phase'', or any uv-variables. The default is ``time''.

Key: yaxis
        yaxis can be antenna gain ``amplitude'' or
        ``phase'', or any uv-variables. The default is ``phase''.

Key: log
        Output log file. Default is the terminal.

Key: device
        Standard PGPLOT device, if plot of yaxis versus xaxis is wanted.
        Default is no plot.

Key: nxy
       Number of plots in the x and y directions. The default is 3,3.

Key: xrange
       The min and max range along the x axis of the plots. The default
       is to autoscale. Note that for "time" should be given in days.

Key: yrange
       The min and max range along the y axis of the plots. The default
       is to autoscale. Note that for "time" should be given in days.

Key: refant
        Reference antenna for gains and uv-variables. If refant.ne.0 then
        the gain of this antenna is set to cmplx(1.,0.). The
        other antenna gains are then relative to the reference antenna.
        The uv-variable value for the refant is subtracted from each of the
        other antennas. Single valued variables, e.g. time, are unchanged.
        The default is to use the original gains and uv-variable values.

Key: refant2
        Reference antenna for cross correlation of gains and uv-variables.
        Compare the gains or uv-variables between antennas by fitting
        	yaxis = slope * xaxis(refant2) + intercept
               D(yaxis) = slope * D(xaxis(refant2)) + intercept
        where D() is the difference between succesive samples.

Key: options
         wrap         Do not unwrap phase.
          rescale      The uv-variables are scaled to the same range as 
        	       the reference antenna. The default is no scaling.
          structure    Replace yaxis with it's structure function:
        		   {yaxis(i+k) - yaxis(i)}**2 
          allan        Replace yaxis with it's Allan variance:
        		   {yaxis(i+2k) - 2*yaxis(i+k) + yaxis(i)}**2 
          quad	       Fit yaxis = a + b*xaxis + c*xaxis**2
        		where a and b are first derived from a linear fit.
        		The fit is written into the log file. E.g.
        		yaxis=tpower xaxis=tpower refant2=4 options=quad
        		results can be used in the task tpgains to correct
        		tpower to a common Tsys scale.