Just wondering if my opponent declares he will use a focus to reduce my attack, does he announce it right before I roll my dice or at the beginning before I pick a weapon and target?

Never mind, I misread the card.

OK, I think I figured it out. In my opinion, this happens in the Roll Attack Dice step. According to the rules reference, the attacker can resolve any card abilities that grant him additional dice during this step, so it seems to be logical, that the opponent's dice reducing abilities are resolved in this step, too.

"When an enemy ship at Range 1-2 is attacking, you may spend a focus token. If you do, the attacker rolls 1 fewer attack die." This vaguely seems to allow it to be at any time from when the attacker declares a weapon and target to the point that they roll dice, but practically, I agree with what Ubul said (and in the case that the attacker has an ability adding or removing attack dice, initiative decides which one activates first), with the stipulation that the attacker does not "intentionally distract or rush an opponent with the intent of forcing a missed opportunity".

Should the attacker declare a target and roll immediately, without giving the opponent time to trigger Kanan, in this case the player with Kanan should still be given the opportunity to trigger his ability, and if so the attacker must roll again with one fewer attack die. This begs the question though, in this situation, should the attacker roll again whether or not the ability is triggered? Or does the defender get the advantage of knowing the attacker's roll before choosing to remove a die {forcing a new roll}?)