Turned-down edge (TDE), shown at the top of Figure 5, is, like astigmatism, less common in commercially made SCTs than in homemade Newtonians. It is still possible, however. The signature of this defect is that diffraction rings on one side of focus are more sharply defined than they are on the other side. TDE happens when the edge of the primary mirror is flat rather than gently sloping to the center, as it should be. During grinding and polishing, the edge has literally been "turned down." Its effect on images can be as bad as that of the other aberrations.
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