Assertion: newtonian telescope's mirror | /r/HasProperty | needed
Facets:
Source sentences:
text: Because light entering the open end of the telescope tube (right side of picture) is reflected back through the tube, a flat diagonal mirror is needed to deliver the image out the side of the tube to an eyepiece.
text: A secondary flat mirror is needed in order to bounce/bend the light at a 90 degree angle so that the focal point is now outside the main path of the primary mirror.