Didn't imperial officers (or general public) know that the Emperor was a Sith?

In Episode IV, everyone seems to believe that Vader is the only one left who can use the Force.

Tarkin: The Jedi are extinct. Their fire has gone out of the universe. You, my friend, are all that's left of their religion.

Of course, they thought the Jedi were all killed, but what about the Emperor?

I realize that he is not even mentioned in ANH, so did nobody know about he being a Sith, or its just the fact that he was not intended to be brought up in the movie at that point?