An analytic technique has been developed to construct the asymptotic representation of the spaced position and frequency coherence function in Markov's diffusive approximation. The technique employs the formalism of the quasi-classic complex paths in an extended complex-valued coordinate space. It allows the construction of the coherency for arbitrary realistic models of the structure function of the fluctuations of the refractive index of the medium of propagation. The technique has been employed to obtain explicit analytic asymptotic solutions for some realistic models of the structure function. For the quadratic structure function the method produces the known rigorous solution in an automatic fashion.