The effects of atmospheric turbulence on imaging systems, and beam projection systems such as laser designators, can be evaluated and the performance predicted in terms of a properly path-weighted value of the turbulence structure constant, Cn2. An operating system for direct field determination of this weighted value with a portable slit scanning telescope and on-line data reduction minicomputer system will be described. This system utilizes a point laser source located in the vicinity of the target and scans the image of that source from the opposite end of the optical path. The on-line computer system combines data, for the system on test, with the data taken on the atmosphere at the instant of test firing, utilizing Fourier and Abel transform techniques, to provide a prediction of behavior of the system on test, or an evaluation of the effects of the atmosphere on that test. The results can be presented in a variety of forms, including immediate hard copy plots of the MTF of the atmosphere, of the overall system being tested, or plots of predicted radial distribution of intensity on target for the system on test. The slit scanning telescope can track a slowly moving target, with internal optical tracking. A gyro platform is under development which can provide operation from shipboard or other unstable platform.