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Unfortunately, there is no telescope or of any type of other optical instrument that is completely free of image aberrations. The perfect telescope does not exist. Even the eye has some aberrations. But it is always possible to develop optical systems which correct for particular aberrations. Often, it is also a matter the observer’s attitude: i.e. whether he accepts an optical system with certain aberrations, or whether he demands a high-end instrument which produces a virtually perfect image.

Astigmatism can be caused by the incident light beam hitting the telescope obliquely (oblique astigmatism). It can also occur due to distortions of the main mirror. But it is often caused by two different curvatures of mirrors or lenses generating different focal lengths. One bundle of rays would then be perpendicular to the other. Astigmatism can be seen in the Airy disk as an image distortion where it is longer in one axis than that perpendicular to it. The aberration can be minimized by stopping down the telescope.