Well darkness limits LOS to 2 hexes, and most (all?) mortars in the game have a minimum range of 2 hexes .. so the only Direct Fire available for a Starshell is going to be exactly 2 hexes away.That's what I figured. It wouldn't make sense to limit a mortar to indirect fire only. Also a unit that fires at night is Spotted to be fired at.
I'd say this would use the normal Direct Fire placement rules.