Missile Turret

You deploy it as usual, in your own watch. You can use its Sortie abilities only in an opponent's sortie phase, after dispatch (in the second window or the final one), while they have crew in the mothership battle attacking you. Never in your own turn, in a watch or before dispatch. It is checked then, not again as it resolves: if every attacker has left by then, it still does what it can (802.12, 802.12d).

Exhaust it to use it. Choose one of your opponent's crew in the mothership battle when you use it; without one, it can't be used. It takes the damage like battle damage, plating first; the damage stays, and a crew left with no hull is destroyed at once. If it has left the battle by the time this resolves, nothing happens (705.7). Only in the sortie phase; it goes into the crossfire. A base can use it in the turn it comes on board (307.4).