the system is a test instrument used to measure the quality of the optical path through the atmosphere . the device measures the atmospheric transverse coherence length , also known as fried &# 39 ; s parameter or the diffraction limited aperture of the atmosphere , and the angle - of - arrival of incident optical radiation . the system consists of a high resolution charge coupled device camera mounted at the focal plane of a telescope and interfaced with a high resolution frame grabber and computer . incident optical radiation is focused by the telescope onto the ccd array in the camera . the picture frame is then transferred to the frame grabber where the computer determines the size of the focussed spot and position of the spot in pixel coordinates , the atmospheric transverse coherence length and the angle - of - arrival . the software required to control the camera and determine the parameters is an integral part of the system .