As long as you order item 260140MT with the picatinny mount, you can use this as a clip-on night vision device. As far as front of behind the optic, that will depend on the type of optic you are using. If you are using a night vision compatible red dot sight, then it would go behind it (between the red dot and your eye). However, if you are using a traditional scope, then it would go in front of the scope.