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)
             OPTION - 1 -- Bayesian computation
                      2 -- Normal (but with weights)
     
     If OPTION=1
     
     F1  --  Lower limit of veiling factor from prior information.
             Normally this will be set equal to 0.
     
     F2  --  Upper limit of veiling factor from prior information.
             If the spectra have been correctly normalised, F2 should
             in theory be no more than 1, however, it does correlate
             with Vsin i and the spectral type. Thus if you use a very
             broadened template, it may want to reach F > 1, and so
             there may well be reason to set F2 > 1. It is unlikely
             that F2 should ever be < 1.
     
     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. Option (1) Cross-correlation if stored is actually 
  equal to (sum W S T)**2/(2 sum W T**2) where S is spectrum and T template
  plus a correction for the known range of the fraction of the spectrum
  between 0 and 1. (2) The same without the correction (3) Stores 
  sum W S T in one block and then sum W T**2 in the next. If the spectra were 
  normalised to have unity continuum which was then subtracted then these 
  cross-correlations can be back-projected to form a `skew map'. The routine 
  does not account for uncertainties in the template spectra. The Bayesian 
  correction factor reduces probability for negative cross-correlation using 
  prior knowledge that the template can only make up a factor f between 0 and 
  1 of the spectrum. Spectra and templates must be correctly scaled.
     
  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