Just to clarify some misinformation circulating on the internet and in-game:
-You NEED to have completed the Second Dream quest
-You NEED to have completed the Sedna Junction and it's requirements, even if you've already unlocked Sedna before Specters of the Rail and the new Solar Map
-You NEED to play through Pluto missions in order to reach the Sedna Junction before you can challenge the Saryn Specter (meaning you must have completed any missions on the white line that leads into the Sedna Junction)
On Orokin Principle Challenges to unlock the Sedna Junction:
-You CAN complete the same challenge type more than once and all attempts will count towards the Sedna Junction requirements (for example: The co-op platform challenge which rewards the mod Coaction Drift spawned on two different occasions for me, both were counted toward the requirement upon completion)
-For some of these you may want at least two players, the platform challenges can be done with two players and the use of specters/eximus/Loki decoy/using your operator's focus ability
-The music challenge room where you play Simon Says does NOT count towards the junction requirement
-These challenge rooms ONLY spawn on Lua, not in the void or on derelicts
-Multiple challenge rooms can spawn in a single mission, the most I've had is two
-Guides for each challenge room can be found here under the header "Halls of Ascension":
http://warframe.wikia.com/wiki/Orokin_Moon-This is also a fantastic, commentary-free video demonstrating how to do each challenge solo (except the co-op platform one), great if you are also trying to complete the "Seven Principles" achievement:
Also, just some notes on the quest itself:
-You will need an archwing to complete part of the quest (Edit: The archwing does NOT need to be fully levelled, you simply need access to one) - Credit: Vr English
-There is a temporary "point of no return," so make sure you come equipped with your best gear
-Having an operator hood equipped will NOT prevent your operator's face from being seen in cutscenes
Good luck! You're in for a treat.