Assertion: collimator's use | /r/HasProperty | balance
Facets:
value: always
type: DEGREE
value: between the mass of the mirrors
type: OTHER
Source sentences:
text: X-ray focusing mirrors were used as early as 1978 (on the Einstein satellite); however, the use of mirrors versus collimators is always a balance between the mass of the mirrors, the size of the spacecraft, and the science goals of the mission.