Astigmatism

When a telescope mirror is ground asymmetrically—so one axis has a shorter focal length than the other—it suffers from astigmatism. This aberration is less common in machine-made CAT mirrors than it is in home-built Newtonian primaries, but it can still happen. Severe astigmatism, seen in Figure 4a through 4c, makes in-focus stars, especially at the edge of the field, look more like crosses than points. If the aberration is less severe, these stars may merely look slightly elongated or enlarged (unlike stars in a miscollimated scope, they will look worse at the field edge than at the center). Stars at the field edge of a "good" scope will also look enlarged or elongated due to the SCT's naturally curved field, but real astigmatism is very obvious in the star test. The intrafocal and extrafocal diffraction patterns are elongated, with the elongations at an angle of 90° to each other. This elongation is the classic symptom of astigmatism. If astigmatism is suspected, the first thing to do is swap eyepieces. If there are still obvious defects in the stars, rotate your head. If the defects—elongation, "spikes," and the like—seem to track head movement, the problem is your eyes, not your scope or eyepiece.

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