Assertion: telescope's element | /r/HasProperty | mirror
Facets:
Source sentences:
text: The idea that the objective, or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope.
text: When this element is a mirror, the result is a three-mirror anastigmat.
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.