docs: def SLIT_MICHELSON(x,g):
      """
      Instrumental (slit) function.
      B(x) = 2/γ*sin(2pi*x/γ)/(2pi*x/γ) if x!=0 else 1,
      where 1/γ is the maximum optical path difference.
      """
      y = zeros(len(x))
      index_zero = x==0
      index_nonzero = ~index_zero
      dk_ = 2*pi/g
      x_ = dk_*x[index_nonzero]
      y[index_zero] = 1
      y[index_nonzero] = 2/g*sin(x_)/x_
      return y