hud head clamp
the idea is that you use the eye liner, the three balls on the centre windscreen post, to ensure your head is in the correct spot.
BUT the HUD has a very small angle of view, so very often a small head movement moves the huds display out of the direct and correct line of sight.
Not normally a major problem, I find the worst problem, is the very slight lack of infinity, and thus a slight re focus is required. Now thats a problem.