At a location that has to remain nameless....we were tasked to hover at specified heights directly overhead a specific smoke stack.....at night. We worked upwards at the specified intervals till several thousand feet above the ground.
We quickly decided to use the FLIR Unit to pick up our cues by slewing it down so it was vertical taking into account the hover attitude. We basically bore sighted the smoke stack using the FLIR. Add that we were using NVG’s in addition to the FLIR made that the most unusual vertical reference maneuver I ever had the misery to do!