One of our music directors will occasionally use the altar as a surface to place an overhead projector on (during practice - outside of Mass), to be able to project music on a screen that is placed behind the altar. Is this permissible?
No, it is not permissible. Cannon 1239, paragraph 1 states: “Both a fixed and a moveable altar are to be reserved exclusively for divine worship and entirely exempt from profane use.” Placing an overhead projector on the altar would definitely constitute profane use.
Fr. Vincent Serpa, O.P.