A method is built that investigates the influence of the atmospheric turbulence over the optical images of cosmic objects, formed in the focal planes of ground-based telescopes. Its main element is the opportunity found for analytic presentation of the influence of turbulence as ''multiplicative correction'' of the images which would be formed if turbulence was missing. Mathematical models are introduced, the descriptions of which allow integrations with elementary quadratures. The method built in this way has wider application and better analytic effectiveness. There have been reached explicit final formulae about the distribution of the average temporal optical intensity in the focal plane of the telescope. The final formulae give the opportunity of easy quantitive confrontation of turbulent expansions of the images with their diffractional expansions. Explicit final expressions about the contrast of the fluctuations (about the distribution of the mean quadratic deviation, compared with the distribution of the mathematical expectation), about the correlational radii of the fluctuations in front of the entrance aperture of the telescope and in its focal plane, are derived.