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Basic Information on invert


Task: invert
Purpose: Transform multi-pointing visibility data into a map
Categories: map making

        INVERT is a MIRIAD task that forms images from visibilities.
        INVERT can form continuum images or spectral line cubes.  It can
        generate images/cubes for several polarisations, as well as
        handling multi-frequency synthesis and mosaicing observations.
        INVERT can also form complex-valued images from non-Hermitian
        data (e.g. holography data).  Appropriate point-spread functions
        can also be generated.

Key: vis
        Input visibility data files.  Several files can be given.
        No default.

Key: map
        Output map (image) file name.  Each output file consists of a
        single polarization/Stokes parameter.  If several different
        pols/Stokes images are being made, then several file names
        should be given.  No default.

Key: beam
        Output beam (point-spread function) file name.  The default is
        not to make a beam.

Key: imsize
        The size of the output dataset.  The default is to image out to
        primary beam half power points. Add 'beam' as the third value
        to specify the image size in primary beam FWHMs (e.g.,
        imsize=2,2,beam).
      . For options=mosaic, an image of this size is made for each
        pointing before a linear mosaic operation is performed.

Key: cell
        Image cell size, in arcsec.  If two values are given, they give
        the RA and DEC cell sizes.  If only one value is given, the
        cells are made square.  The default is about one third of the
        resolution of the resultant images. Add 'res' as the third value,
        to specify the the cellsize in terms of the number of pixels per
        resolution element (e.g., cell=5,5,res)

Key: offset
        When not mosaicing, this gives the sky position to shift to the
        center of the output images.  The position is specified as an
        offset (in arcsec) from the observing center.  The default is to
        perform no shifting.
 
        When mosaicing, this gives the sky coordinate (RA and DEC) of
        the reference pixel in the imaging process.  The value can be
        given in the form hh:mm:ss,dd:mm:ss, or as decimal hours and
        degrees.  INVERT applies appropriate shifts to make this
        location fall on a pixel.  The default is a central observing
        center.

Key: fwhm
        This determines a gaussian taper to apply to the visibility
        data.  It specifies the FWHM of an image-domain gaussian --
        tapering the visibility data is equivalent to convolving with
        this image-domain gaussian.
 
        Either one, two or three values can be given. For one or two
        values this speciefies the FWHM in arcsec in the RA and DEC
        directions.  If only one value is given, the taper is assumed
        to be symmetric.  If three values are given, they specify the
        FWHM major and minor axis in arcsec and the position angle in
        degrees. The default is no taper.
 
        The signal-to-noise ratio will be optimised in the output image
        if this parameter is set to the FWHM of typical image features
        of interest.
 
        If you are more accustomed to giving this parameter in the uv
        plane (as AIPS requires), then:
          fwhm(image plane) = 182 / fwhm(uv plane)
        where the image plane fwhm is measured in arcseconds, and the uv
        plane fwhm is measured in kilowavelengths.

Key: sup
        Sidelobe suppression area, given in arcseconds.  This parameter
        gives the area around a source where INVERT attempts to suppress
        sidelobes.  Two values (for the RA and DEC directions
        respectively) can be given.  If only one value is given, the
        suppression area is made square.  The default is to suppress
        sidelobes in an area as large as the field being mapped.
 
        Note that uniform weighting can produce images with spuriously
        high noise, especially for mfs imaging. It is recommended
        to use robust~0.5 if sup is non zero or unset.
 
        The suppression area is essentially an alternate way of
        specifying the weighting scheme being used.  Suppressing
        sidelobes in the entire field corresponds to uniform weighting
        (so the default corresponds to uniform weighting).  Natural
        weighting gives the best signal to noise ratio, at the expense
        of no sidelobe suppression.  Natural weighting corresponds to
        SUP=0.  Values between these extremes give a tradeoff between
        signal to noise and sidelobe suppression, and roughly correspond
        to AIPS "super-uniform" weighting. [A better way to move between
        these extremes is to leave sup unset and vary the robust
        parameter from -2 to 2.]

Key: robust
        Brigg's visibility weighting robustness parameter.  This
        parameter can be used to down-weight excessive weight being
        given to visibilities in relatively sparsely filled regions of
        the $u-v$ plane when using uniform weighting.  Most useful
        settings are in the range [-2,2], with values less than -2
        corresponding to very little down-weighting, and values greater
        than +2 reducing the weighting to natural weighting.
 
        Sidelobe levels and beam-shape degrade with increasing values of
        robustness, but the theoretical noise level will also decrease.
 
        The default is no down-weighting (robust=-infinity).

Key: line
        Standard line parameter, with the normal defaults.  In
        particular, the default is to image all channels.  See the help
        on line for more information.
        The line parameter consists of a string followed by up to
        four numbers, viz:
 
          linetype,nchan,start,width,step
 
        where 'linetype' is one of "channel", "wide", "velocity" or
        "felocity".

Key: ref
        Line type of the reference channel, specified in a similar to
        the line parameter.  Specifically, it is in the form:
          linetype,start,width
        Before mapping, the visibility data are divided by the reference
        channel.  The default is no reference channel.

Key: select
        This allows a subset of the uv data to be used in the mapping
        process.  See the Users Manual for information on how to specify
        this parameter.  The default is to use all data.

Key: stokes
        Standard polarisation/Stokes parameter selection.  See the help
        on stokes for more information.  Several polarisations can be
        given.  The default is "ii" (i.e. Stokes-I, given the assumption
        that the source is unpolarised).

Key: options
        This gives extra processing options.  Several options can be
        given (abbreviated to uniqueness), and separated by commas:
          nocal     Do not apply gains table calibration to the data.
          nopol     Do not apply polarisation leakage corrections.
          nopass    Do not apply bandpass table calibration to the data.
          double    Normally INVERT makes the beam patterns the same
                    size as the output image.  This option causes the
                    beam patterns to be twice as large.
          systemp   Weight each visibility in inverse proportion to the
                    noise variance.  Normally visibilities are weighted
                    in proportion to integration time.  Weighting based
                    on the noise variance optimises the signal-to-noise
                    ratio (provided the measures of the system
                    temperature are reliable!).
          fsystemp  Like systemp, but use frequency dependent Tsys.
                    You need to run atrecal before invert to create the
                    systempf variable containing the Tsys spectrum.
                    Atrecal requires autocorrelations to be present.
                    This option only works in combination with the
                    mfs option.
          mfs       Perform multi-frequency synthesis.  The causes all
                    the channel data to be used in forming a single map.
                    The frequency dependence of the uv coordinate is
                    thus used to give better uv coverage and/or avoid
                    frequency smearing.  For this option to produce
                    useful maps, the intensity change over the frequency
                    band must be small.  Set the 'line' parameter to
                    select the channels that you wish to grid.
          sdb       Generate the spectral dirty beam as well as the
                    normal beam, when MFS processing.  The default is
                    only to create the normal beam.  If the spectral
                    dirty beam is created, this is saved as an extra
                    plane in the beam dataset.
          mosaic    Process multiple pointings, and generate a linear
                    mosaic of these pointings. For single pointings
                    to be combined with linmos you can use this to
                    specify a common reference position with the
                    offset parameter. Observations using OTF mosaicing
                    always need to specify this to ensure the moving
                    beam is handled properly.
          imaginary Make imaginary image for non-Hermitian data
                    (holography).
          amplitude Produce a image using the data amplitudes only.  The
                    phases of the data are set to zero.
          phase     Produce an image using the data phase only.  The
                    amplitudes of the data are set to 1.
          sin       Label the output map and beam as a SIN projection.
                    Default is NCP unless non-east-west baselines are
                    present or the field centre is within 3 deg of the
                    celestial equator (because NCP blows up near the
                    equator).  Note that this option simply changes
                    ctype1 and ctype2 in the header, the translation
                    only being correct to first order about the field
                    centre.  A similar result could be obtained by
                    running 'puthd' on the output map, e.g.
                      puthd in=<map>/ctype1 value=RA---SIN
                      puthd in=<map>/ctype2 value=DEC--SIN
                    and likewise for the beam
          ncp       Force invert to use the NCP projection even when
                    significant non E-W baselines are present.
                    Use with care..

Key: mode
        This determines the algorithm to be used in imaging.
        Possible values are:
          fft    The conventional grid-and-FFT approach.  This is the
                 default and by far the fastest.
          dft    Use a discrete Fourier transform.  This avoids aliasing
                 but at a hugh time penalty.
          median This uses a median approach.  This is generally robust
                 to bad data and sidelobes, has a even larger time
                 penalty and produces images that cannot be deconvolved.
        NOTE: Dft and median modes are not supported with
        options=mosaic.

Key: slop
        NOTE: This parameter should be used with caution!  See the Users
        Guide for more information on its applicability.
 
        When forming spectral cubes, INVERT normally insists that all
        channels in a given visibility spectrum must be good before
        accepting the spectrum for imaging.  This keyword allows this
        rule to be relaxed.  It consists of two parts: a tolerance and
        a method for replacing the bad channels.
 
        The tolerance is a value between 0 and 1, giving the fraction of
        channels that INVERT will tolerate as being bad before the
        spectrum is totally discarded.  The default is 0, indicating
        that INVERT will not tolerate any bad channels.  A value of 1
        indicates that INVERT will accept a spectrum as long as there is
        at least one good channel.
 
        The replacement method is either the value `zero' or
        'interpolate', indicating that the bad channels are either to be
        replaced with 0, or to be estimated by linear interpolation of
        two adjacent good channels.  See the Users Guide for the merits
        and evils of the two approaches.  The default is 'zero'.
 
Revision: 1.23, 2017/07/27 05:46:17 UTC

Generated by miriad@atnf.csiro.au on 27 Jul 2017