According to the Declaration Of A New Order (the speech given by Palpatine in ROTS declaring that the Republic is now an empire), Palpatine led everyone to believe that Dooku was one of the Jedi and that they were using him in attempt to take over the Republic, and perhaps even that the Jedi were behind the CIS movement. (see the full text of Sidious's speech here) Sith were also widely seen as evil (though thought to be extinct until shortly before all of this). These facts suggest that no one (aside from his apprentices, the Jedi, etc) knew that Palpatine was actually a Sith lord. Pretty much everyone saw him as a hero who defeated the Jedi, and they may have even thought that the Jedi were allied with the Sith (because of Dooku being on the Jedi's side according to Sidious). However, in the OT, it is clear that at very least some of the high ranking imperials know that Vader and Sidious are Sith lords. Vader openly uses the Force and his lightsaber. How does this all fit together? True, the above was a meeting of high ranking imperials and perhaps the truth was kept from the general public, but the name 'Darth Vader' alone, and the fact that he walks around with a lightsaber, make it more than obvious that he is a Sith Lord. Did the empire simply have too much power by the time the truth got out and people felt like they couldn't do anything about it? Even if that is what happened, why did the imperials (stormtroopers aside since they're 'programmed' to obey) all remain loyal?