bckfil

 BCKFIL N1 N2 N3 FWHM -- Applies filter that can be used prior to 
                      back-projection in order to compute filtered
                      back-projection used in Doppler tomography.

 The filter applied is ABS(S)*EXP(-(S/W)**2/2) where S is the frequency with
 the Nyquist frequency of one cycle/2 pixels as the maximum. W is a user
 defined parameter (see below) that suppresses high frequencies and leads
 to a blurring of the final image.

 The filter is applied to the entire spectrum. You may want to rebin before 
 using it if you are only interested in a small region of your spectra,
 although it is a reasonably fast operation.
 
 Parameters:
            N1   -- First spectrum
            N2   -- Last spectrum
            N3   -- First output slot
            FWHM -- FWHM of window to taper off high frequencies. It is
                    scaled in terms of the Nyquist frequency which is the 
                    highest useful frequency. Thus FWHM >>1 will have little
                    effect and may be OK if you have good data whereas for
                    noisy data you may wish to choose a FWHM < 1. You
                    should probably start with a high value and decrease it
                    if the result looks very noisy. Even quite noisy data 
                    does not seem to require FWHM < 0.5.

This command belongs to the classes: binary and arithematic .