Up Periscope is a 1959 World War II submarine film directed by Gordon Douglas. We often think the phrase “up periscope” is used when the skipper of a submarine wants to check his surroundings. I thought as much, but then realised that modern submarines don’t have a traditional periscope. In fact, they have two “periscopes”, or rather photonics masts which house instruments for not just observing the visual environment, but for collecting a range of scientific data.
“To raise either scope on a 688i boat (3rd generation Los Angeles class fast attack submarine), you just have to rotate a metal ring 
that actuates via servo the hydraulic pump to lift or lower your desired scope. And it’s not just the captain who does this. (The guy has other things to do than just look through a scope). Any officer who is qualified Officer Of the Deck does this as part of their job, and they announce whichever scope they’re raising or lowering as a courtesy to everyone else is control. Example; “Raising #2 Scope!”
This announcement lets the ships control party (Dive, Helmsman, Planes-man, Chief of the Watch) know that the ship cannot descend below a certain depth because that would limit the scopes visual / sensor abilities. Also, the boat cannot go beyond a certain speed with a scope in the air as driving too fast could potentially damage it due to hydrodynamic drag.
Radio, Fire Control and the Quartermaster appreciate knowing that there’s a scope in the air because the scope does more than just see. It’s packed with sensors that feed information to either of the three watch stations. Information like the presence of another ocean-going vessel that projects its own RADAR signal. Depending on that RADAR signal strength, Radio uses it as a proximity warning that helps determine whether to inform Control if we should dive due to fear of being run over by the otherwise unassuming ocean liner. (It’s happened before.) Radio will also use this opportunity to clear the broadcast of any necessary message traffic both sent and received. Fire Control always like having tech in the air because it increases their tactical awareness. The Quartermaster (navigation plot) appreciates this too because they can use a scope to help realign with satellite navigation and reset the RLGN’s for further submerged navigation.”
In my new novel, I had to avoid the “up periscope” cliche, yet give readers a sense of this classic action. “Raising #2 Scope” helped.