Assertion: newtonian telescope's mirror | /r/HasProperty | exposed to dust
Facets:
Source sentences:
text: Reflector telescopes are mainly open telescopes, meaning that the mirrors are exposed to the air, humidity and dust.
text: Reflector telescopes are more fragile than other types of telescopes and need periodic readjustment of the telescope mirrors (a process called collimation) and, because they are open on the top (Newtonian reflector), the mirrors are exposed to dust and dirt and therefore need periodic cleaning.