UV_PREVIEW Algorithm

        UV_PREVIEW computes for Ntapers different tapers  the  spectrum  towards
    the  phase  center. The taper ranges are determined from the available base-
    line lengths and telescope diameter. Ntapes is controlled by  the  value  of
    Key TAPER.

    These  spectra  are  then smoothed by averaging several channels (1,4,16 and
    64) to be sensitive to different linewdiths. The nummber of smoothed spectra
    per taper is controlled by the value of Key SMOOTH.

    For each spectrum, UV_PREVIEW then attempts to figure out if there  is  line
    emission  and the line-free channels to define the continuum level.  This is
    based on the histogram of the intensity distribution of  all  channels.  The
    most likely value and the noise level is derived from this histogram. An it-
    erative  scheme,  blanking out of range (presumably line emission) channels,
    is used for this to converge towards a Gaussian  histogram,  which  normally
    represents the noise distribution around the continuum level.

    As  a  last  step,  blanked  channels are accumulated in a list of channels,
    which thus contain possible line emission at  any  of  the  Ntapers  angular
    scales and Nsmooth spectral resolutions.