xcor

  XCOR N1 N2 N3 N4 N5 SLO SHI TAPER FILE OPTION [F1] [F2] SET NAME -- 
     Cross-correlates spectra. See below for more details.
     
  Parameters:
     N1   -- First object spectrum
     N2   -- Last object spectrum
     N3   -- First template spectrum
     N4   -- Last template spectrum
     N5   -- First slot for cross-correlations. They are stored
             as spectra with 5000 A corresponding to 0 km/s.

     SLO  -- First shift in pixels to compute. 
     SHI  -- Last shift in pixels to compute. A search will be
             made for a REAL header parameter called 'XCOR_SHIFT'
             and if found SLO and SHI will be assumed relative to 
             this. If an arc is available, it is assumed that 
             'XCOR_SHIFT' has units of 'km/s', otherwise pixels.
             This should help pick out the right peak in cases of
             low signal-to-noise especially when binary motion is
             significant. The value of XCOR_SHIFT is converted to 
             the nearest equivalent integer pixel shift and then
             cross-correlation is carried out from SHIFT+SLO to
             SHIFT+SHI. The range SLO to SHI can then be reduced 
             to an amount consistent with the need to bracket the 
             maximum.
            
     TAPER - Fraction to taper at ends of spectra. This reduces
             end effects.
     FILE  - ASCII file to store results (blank to ignore)
     
     SET   - Yes to set mask of pixels in object (not template)
     NAME  - Name of header item to store radial velocity in.
             (uncertainty will go to 'NAME_err')
     
  Carries out standard computation of cross-correlation, interpolating
  over regions masked out of analysis. Maximum cross-correlation located
  by parabolic approx to three points at maximum. Purely statistical
  uncertainty computed. 
     
  Scaling is probably best done by first fitting a constant to a region
  of the spectra of both templates and targets and then dividing this 
  through. Note that a spline or higher order poly is not desireable
  because it might change the relative line strengths at different 
  wavelengths. Following normalisation then one should subtract a fit
  to the continuum. Another possible method is to apply a band-pass
  filter with 'bfilt', especially to filter out longer term variations.
     
  The velocities that xcor produces are only accurate if prior to
  using it you have rebinned both targets and templates onto an identical
  velocity scale with 'vbin'.
     
     

This command belongs to the class: measurement