In  interactive  mode, the user can graphically view the results
        of the mean calculation.  There are two  views  available.   The
        default  view  covers  just  the  region of the spectrum that is
        averaged together.  The mean is indicated by a solid line  drawn
        across  the  graph.   The standard deviation is indicated by the
        vertical error bar around the mean in the center of  the  graph.
        If  points  in  this region have been rejected, those points are
        indicated by small crosses on those points.
        
        The second view, invoked by the 'f' command, is the "full" view.
        Here,  the  whole  spectrum  is  displayed.  Again, the mean and
        standard   deviation   are   indicated   as  above.    Also,   a 
        horizontal error  bar,  towards  the  bottom   of   the   graph,
        indicates  the  region  that  is  included  in  the average.  No
        rejected pixels  are indicated in this view.
        
        The following single key stroke commands are available:
        
        :   See below
        
        ?   The help file for the interactive mode of centerflux.
        
        I   Discontinue  interactive  mode.   The result for the current
            spectrum is used.  If there  are  any  more  spectra  to  be
            processed,     they     will     be     calculated,    using  
            whatever  modifications   were   made   to   the  parameters
            up  to  this point.   There is no way  of  getting  back  to
            interactive mode once left in this manner.
        
        c   Redefine the center of the pixel region to  be  averaged  to
            where   the  graphics   cursor   is   located.    Also   see
            the "center" colon command below.
        
        f   Toggle between the "full"  view  of  the  spectrum  and  the
            "restricted" view of just the region that is averaged.
        
        p   Redefine  the  region  over which is averaged.  Two define a
            new region, move the graphics cursor to wherever the  region
            should  begin and hit 'p'.  To define the end of the region,
            move the cursor to where the region should end and  hit  'p'
            again.  See also the "start" and "end" colon commands below.
        
        q   Indicate  that the current result is fine and continue on to
            the next spectrum.
        
        r   Redraw the graph.
        
        w   Toggle    between   wavelength   and   pixel    coordinates. 
            By  default,    the    x-axis   is   plotted    using    the 
            wavelength   information   from   the    wavelength    file.  
            Hitting  'w'  will switch to a pixel coordinate  system  and
            back.
        
        The following colon commands available are:
        
        center value
            Display  or set the center pixel.  If no value is specified,
            the current center is displayed.  If there is  a  value,  it
            is  taken  as  the  pixel  which  to place the center of the
            region   to   be   averaged.    By   default,   center    is 
            INDEF,  indicating  to  use whatever the center pixel of the
            current spectrum is.  This value is written  back   to   the
            "center" task parameter when the task is finished.
        
        end value
            Display  or set the end pixel of the region.  If no value is
            specified, this  displays  the  current  end  pixel  of  the
            region.   Note  that  both  start  and end positions must be
            specified in order for them to take  effect.   If  not,  the
            'center'  and  'size'  parameters define the pixel region to
            be averaged.  This value is written back to the  "end_pixel"
            parameter when the task completes.
        
        low_reject value
            Display  or set the low_reject sigma.  This value is written
             back to the "low_reject" parameter when the task completes.
        
        high_reject value
            Display  or  set  the   high_reject   sigma.    This   value
            is  written   back  to  the "high_reject" parameter when the
            task completes.
        
        iterations value
            Display  or  set  the   number   of   rejection   iterations
            to   perform.     This   value   is   written  back  to  the 
            "iterations" parameter when the task completes.
        
        size value
            Display or set the size of  the  region  to  average.   This
            value  is written back to the "size" parameter when the task
            completes.
        
        start value
            Display or set the start pixel of the region.  If  no  value
            is  specified,  this displays the current start pixel of the
            region.  Note that both start  and  end  positions  must  be
            specified  in  order  for  them to take effect.  If not, the
            'center' and 'size' parameters define the  pixel  region  to
            be    averaged.     This    value    is    written  back  to
            the "start_pixel" parameter when the task completes.