Observers that are used for deep-space exploration are often deployed without any defensive systems, but when used in a combat zone they are equipped with a micro-cloaking filed to hide them from the enemy. Like the Arbiter, the Observer does not require any additional energy to maintain the field.

I love Observers. Good and cheap, they let you survey areas without risking the lives of your units, as well as seeing any burrowed or cloaked units. Use these for defense and in every attack you launch, so you don't get killed by Lurkers, Wraiths, or Dark Templar.