A simple geometrical method to measure the aberration of
astigmatism present in the wavefront that emerges from a telescope is presented. The
method is based on the analysis of the external contour of the image of
a slightly defocused star. An expression elliptical edge is obtained, which
links the rms value of
to the geometric parameters of the ellipse. This
expression is tested as a function of introduced defocus and astigmatism
aberration in telescopes of the San Pedro Martir Observatory. It is shown that the method
gives comparable results to wavefront tests, being capable of measuring
astigmatism values of
and larger without the need for auxiliary optics.
© 2007 Optical Society of America
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