Assertion: telescope's element | /r/HasProperty | both lenses
Facets:
Source sentences:
text: These elements can be both lenses and mirrors, but since multiple surfaces are involved, achieving good aberration correction in these systems can be very complex.
text: The focal ratio of a telescope is derived from dividing its focal length (how long the telescope is) by the width of its largest optical element (either its lens for a refractor or its primary mirror for a reflector).