function rmsk = get_radial_filter(r0)
     global NFFT;
     N          =   4;
     r0         =   NFFT/r0;
     BW         =   r0*1.75-r0/1.75;
     [x,y]      =   meshgrid(-NFFT/2:NFFT/2-1,-NFFT/2:NFFT/2-1);
     r          =   sqrt(x.^2+y.^2);
     num        =   (r*BW).^(2*N);
     den        =   (r*BW).^(2*N)+((r.^2-r0.^2)).^(2*N);
     rmsk       =   sqrt(num./den);
