FUNCTION FIR_DESIGN,S,M
;+
;h=FIR_DESIGN(S,M) computes the FIR coefficients for a filter of length M
;that will approximate the frequency response S.
;
;S is a vector of length N/2. The frequency response is constructed
;to make S symmetrical about the origin.
;
;H. Rhody
;May, 2002
;-
N=N_Elements(S)
R=[S,0,Reverse(S[0:N-2])]
w=0.54-0.46*cos(2*!pi*findgen(M+1)/M)
h=Shift(FFT(R,/Inverse),M/2)
h=h[0:M]*w
Return,float(h)
END