Abstract
In perpendicular illumination, axicons give a focal line of large depth of focus and narrow, well-known transverse width. In oblique illumination, however, the focal line is broadened and its shape changed. In this contribution, we use the method of stationary phase to characterize the focal segments for oblique illumination in a compact form, which both yields the width of the focus and reveals it to be asteroid-shaped. These results are compared to those for a diffractive axicon of elliptically shaped phase-step pattern, which are found to be slightly squeezed asteroids. It is shown that the changes in the focus are caused by off-axis astigmatism for oblique illumination, and by on-axis astigmatism for the elliptic axicon.
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