Assertion: newtonian telescope's mirror | /r/HasProperty | set perpendicular
Facets:
value: to the tube
type: LOCATION
value: accurately
type: MANNER
Source sentences:
text: There is a transparent spot in the center of the mirror, used as a peephole, and the mirror is set accurately perpendicular to the tube (there are other devices called "autocollimator" too - if you search the net for more info, don't be confused).