I need help. I need a way to (using optics) build a rangefinder that I could use to aim my air cannon over long distances. How would I judge the distance away from my target? Could I use lasers? Mirrors? Lenses? And once I judge how far away my target (to be drenched with a water balloon) is, then what? How will I know what degree of elvation I should angle my cannon at? Is there some mathmatical formula I could use to make a chart that I could just look at and adjust my gun? How could I build this?