code: def SLIT_DIFFRACTION(x,g):
      
      y = zeros(len(x))
      index_zero = x==0
      index_nonzero = ~index_zero
      dk_ = pi/g
      x_ = dk_*x[index_nonzero]
      w_ = sin(x_)
      r_ = w_**2/x_**2
      y[index_zero] = 1
      y[index_nonzero] = r_/g
      return y
docs: Instrumental (slit) function.