Characterization of alignment using measurements of the sine condition

An experimental test for violations of the sine condition is presented. This test is particularly useful for identifying the state of alignment of an imaging system because it provides a direct measurement of the linear astigmatism (astigmatism that varies linearly with field) in a system using only on axis measurements. The concept of the test is explained from the perspective of both geometrical optics, using the sine condition, and wave optics. In addition, the results of an experimental proof of concept are presented. This experiment shows good agreement between the measured and predicted results.