Interferograms of window wavefront deformations as a measure of angular deviations to a line of sight.

An interferogram of the wavefront deformation to a flat wave caused by an aircraft or spacecraft window contains all the error contribution effects of the window. The interferogram may be analyzed to determine the slope of this wavefront at a given point, which yields the angular deviation to a line of sight through the point. The interferogram as used in this study is a two-dimensional photo representing the three-dimensional relationship of the transmitted wave relative to the flat reference wave. The progression from light to dark fringes is the progression of the third dimension normal to the photo, and the fringes are thus similar to contour lines on a topographic map. To demonstrate the ability to obtain interferograms, to analyze interferograms, and to achieve sufficient accuracy, an aircraft window and spacecraft window were studied in an experimental test. The deviations to a line of sight were derived from transmitted wave interferograms obtained in the laboratory and were also measured precisely by an autocollimator system. The standard deviation of the transmitted wave deviations derived from interferograms relative to directly measured deviations was 0.7 sec of arc, indicating that the transmitted wave interferogram method is a good tool for window error determination.