Hello, When an experimentator knows his set-up, how does he know which operator to use (if he were to do theoretical calculations for prediction)? Is the distinction always clear? E.g.: is the distinction between a position and a momentum measurement apparatus always clear? I can imagine an experimentator using a time-of-flight measurement to measure momentum, but the measurement itself is for position, so can the experimentator choose which operator he uses (of the two)? On the other hand, if the distinction is always clear (in which case--correct me if I'm wrong--time-of-flight measurements are not "allowed"), can you name a "pure" impulse measurement?