Documentation on mfspin

Task: mfspin
Purpose: Compute spectral index from a MFCLEAN model.
Categories: deconvolution

        MFSPIN is a MIRIAD task which computes spectral index and possibly
        rotation measure images, given models produced by MFCLEAN.  It
        does this by convolving both the flux and "flux time spectral index"
        planes of the models by a Gaussian, and then performing a division.

        It has two modes of operation.  One is to give a single input model
        (Stokes I or V even) which consists of two planes.  MFSPIN will then
        compute the spectral index image from that.   The second mode 
        requires both Stokes Q and U models.  MFSPIN will then compute
        a linearly polarized intensity spectral index and optionally a 
        rotation measure image.

Key: model
        The model produced by MFCLEAN. No default. Normally this will be
        a single image such as a model for Stokes I or V. However, you
        can also give it two models, one for Stokes Q and one for Stokes U
        (see the discussion above).

Key: beam
        The input dirty beam. This is only required to determine the beam
        size. If the fwhm parameter is given, the beam will not be used.

Key: clip
        The division clip level. One or two numbers can be given. If two
        values are given, then flux pixel values in this range are flagged
        as bad before the division (but after the convolution!). If a single
        value is given, then for Stokes I images (or images for which the
        Stokes parameter is unknown),  all pixel values less than the
        specified clip are flagged. If one value is given and the image is
        of Stokes Q,U or V, then pixels are flagged when their absolute value
        is less than the clip level. The default is 0.

Key: fwhm
        The size, in arcsec, of the gaussian beam to convolve with.
        This will normally be two numbers, giving the
        full-width at half-maximum of the major and minor axes of the
        gaussian. If only one number is given, the gaussian will have
        equal major and minor axes. If no values are given, they are
        computed by fitting a gaussian to the given dirty beam. Note
        that the fitting routine will probably give different values to
        the AIPS MX and APCLN tasks. Generally the value computed by
        MFSPIN is to be preferred to the APCLN and MX values.

Key: pa
        The position angle, in degrees, of the gaussian,
        measured east from north. The default is determined from the dirty
        beam fit (The value for PA is ignored, if no value is given for
        FWHM).

Key: options
        Extra processing options. Several can be given, separated by
        commas. Minimum match is used. Possible options are:
          pbcorr   Correct the spectral index for primary beam effects.
                   By default, this is not performed.

Key: out
        The output image names. No default. Normally this will be a single
        output, being the spectral index. If the input consists of a Q and a
        U model, the output can be two names. The first is the linearly
        polarized intensity spectral index image, and the second will receive
        an estimate of the rotation measure (units of rad/m**2).