docs: def fun_wv(xchannel, crpix1, crval1, cdelt1):
      """Compute wavelengths from channels.
  
      The wavelength calibration is provided through the usual parameters
      CRPIX1, CRVAL1 and CDELT1.
  
      Parameters
      ----------
      xchannel : numpy array
          Input channels where the wavelengths will be evaluated.
      crpix1: float
          CRPIX1 keyword.
      crval1: float
          CRVAL1 keyword.
      cdelt1: float
          CDELT1 keyword.
  
      Returns
      -------
      wv : numpy array
          Computed wavelengths
  
      """
      wv = crval1 + (xchannel - crpix1) * cdelt1
      return wv