Assertion: eyepiece's field | /r/HasProperty | limited by secondary astigmatism
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text: 4 - Monocentric - shown is the modern form of this eyepiece, but performance level is similar to the original - maintains excellent correction even at f/5, but field is limited by secondary astigmatism, exploding as field approaches 15 degrees; since it has a significant higher order aberration component, as well as primary coma, its aberration load diminishes more quickly with slower focal ratios than in eyepiece types limited mainly by primary astigmatism.