Basic Information on varmap

Task: varmap
Purpose: Image uvdata versus selected x-axis and y-axis uv-variables.
Categories: image conversion, analysis.

VARMAP makes a Miriad image of uvdata or single dish data
versus selected x-axis and y-axis uv-variables.
Data is averaged into an image with the given cell and imsize.
No interpolation into adjacent pixels is made.
E.g. VARMAP can map autocorrelation data versus dra and ddec. 
Related tasks:
        UVCAL options=holo replace [u,v] with [dazim,delev]
        INVERT convolves uvdata into a grid and makes a 2D FFT.

Key: vis
The input uv-dataset name. No default.

Key: select
This selects which visibilities to be used. Default uses
all the data. See the Users Guide for information about
how to specify uv data selection.
e.g. 'select=ant(1)(5),-time(00:00,01:09)'

Key: line
Linetype of the data in the format line,nchan,start,width,step
line can be `channel', `wide' or `velocity'.
Default is channel,0,1,1,1, which uses all the spectral
channels. For line=channel, select only one spectral window
if the channels are not contiguous in velocity.
Flagged data are not used.
e.g. line=channel,6,1,15,15 makes the 6 band averages for CARMA.

Key: xaxis
x-axis uvvariable used for x-axis.
An optional second argument gives the index of the uvvariable.
The default index=1.
The default units are defined in an appendix of the Users Guide.
xaxis can also be 'u' or 'v' (nanosecs). Default xaxis=dra,1
e.g. xaxis=dazim,1

Key: yaxis
y-axis uvvariable used for y-axis.
An optional second argument gives the index of the uvvariable.
The default index=1.
The default units are defined in an appendix of the Users Guide.
yaxis can also be 'u' or 'v' (nanosecs). Default yaxis=ddec,1
e.g. yaxis=delev,1

Key: zaxis
Visibility 'amplitude', 'phase', 'real', or 'imaginary'.
Default zaxis=amplitude
No calibration is applied by VARMAP. 'amplitude' is OK on uncalibrated
data, but we need to calibrate the data, e.g. using UVCAL
w.r.t. the (0,0) offset to get  'real', 'imag' or 'phase' correct.
This also could also be an option in varmap to save a
pre-calibration step.

Key: out
Output image. No default.

Key: imsize
The size of the output image x-axis and y-axis. If only one value is
given it is used for both axes. The 3rd axis is
determined by the number of channels in the selected linetype.

Key: xcell
Image pixel size along x-axis.
No interpolation into adjacent pixels is made. Use the same pixel
size as any gridding of the uvvariable. e.g. the dazim and delev
step size used to aquire pointing data.
An optional second argument causes conversion of the units. 
Possible values for the units (a minimum match algorithm is used)
are "arcsec", "arcmin", "degrees" and "hours".
No default. e.g. xcell=30,arcsec

Key: ycell
Image pixel size along xaxis.
No interpolation into adjacent pixels is made. Use the same pixel
size as any gridding of the uvvariable. e.g. the delev step used
to aquire pointing data, or the delay step in a delay search etc.
An optional second argument causes conversion of the units. 
Possible values for the units (a minimum match algorithm is used)
are "arcsec", "arcmin", "degrees" and "hours".
Default is to use the same value and units as xcell.

Key: options
This gives extra processing options. Several options can be given,
each separated by commas. They may be abbreivated to the minimum
needed to avoid ambiguity. Possible options are:
   'sum'      Sum the data in each pixel. Default is to average.

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