In the movies, Luke Skywalker only knows the name "The Emperor". He never even mentions "Darth Sidious" or "Emperor Palpatine". Surely Luke Skywalker knows that the Emperor is Darth Vader's master. But does he know that the Emperor is also Darth Sidious?

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    Or possibly this one: Was Luke aware the Emperor was a Sith Lord? (not about the name specifically, but presumably people would know definitively that anyone with the title "Darth" would be a Sith, so one conclusion would follow the other)
    – tobiasvl
    Commented Jun 8, 2017 at 19:50