Assertion: eyepiece's quality | /r/HasProperty | desired
Facets:
value: in order to shorten the focal ratio
type: PURPOSE
value: commonly
type: DEGREE
Source sentences:
text: The purpose of the correctors in all cases is to improve the off axis optical quality of the telescope, particularly coma and curvature of field, which is commonly desired in order to shorten the focal ratio and increase the width of field.