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.

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