Scenario:

Player takes the Adeptus Astra Telepathica background, but chooses something OTHER than mystic as their role (Chirurgeon in this case).

Later, they decide they want to spend the xp to gain the Psyker elite advance.

Does the character count as a rogue?

Under the AAT rules box (Pg 50) It says that "If a character takes the Psyker elite advance during character creation, he also gains the Sanctioned trait". This implies that past character creation, even though the character has the correct background, they're still not sanctioned if they take the advance later.

However, under the Psyker elite advance box (Pg 90) it states "Characters without the AAT background who gain the Psyker elite advance are not sanctioned." While this makes sense, they've omitted the "during character creation" requirement. what about the above scenario, where the person IS from AAT, but takes the advance later?

So basically, take the 3 scenarios.

AAT background, takes mystic. Gains psyker and sanctioned traits. Pretty sure there's no argument here.

AAT background, doesn't choose mystic, but uses part of their 1000 starting xp to buy the psyker advance. Sanctioned or not sanctioned?

AAT background, doesn't pick up mystic or use 1000 starting xp, goes through first mission, then buys psyker. Sanctioned or not sanctioned?

The fluff the character has in mind at the moment is that they're currently still just discovering their powers, that they underwent the training, yet they haven't developed them to the point where they're usable. Through their experiences with the Inquisition, they then finally get in touch with their mind and learn how to use it. I'm just trying to figure the best way to represent this without any kind of shenanigans regarding the witch hunters.

AAT background, doesn't choose mystic, but uses part of their 1000 starting xp to buy the psyker advance. Sanctioned or not sanctioned?

Sanctioned.

AAT background, doesn't pick up mystic or use 1000 starting xp, goes through first mission, then buys psyker. Sanctioned or not sanctioned?

This could be played either way and I'd personally leave it up to the player. For Sanctioned, I'd say that during their time at the AAT they were a known psyker, but of such a low power level as to not really have any psychic abilities, but still made to go through the process. For Unsanctioned, maybe they flew under the radar and used their knowledge of how the AAT operates to avoid detection (maybe not even knowing they were a psyker).

There's no correct answer.

The fluff the character has in mind at the moment is that they're currently still just discovering their powers, that they underwent the training, yet they haven't developed them to the point where they're usable. Through their experiences with the Inquisition, they then finally get in touch with their mind and learn how to use it. I'm just trying to figure the best way to represent this without any kind of shenanigans regarding the witch hunters.

Some Inquisitors are totally okay with this kind of ability. Others aren't. You can use this as a plot hook - either the player must hide their abilities from their own Inquisitor, or their boss is protecting them from the more Puritan elements within the Inquisition.

The short answer is go with whichever is more interesting.

AAT background, takes mystic. Gains psyker and sanctioned traits. Pretty sure there's no argument here.

Sanctioned.

AAT background, doesn't choose mystic, but uses part of their 1000 starting xp to buy the psyker advance. Sanctioned or not sanctioned?

Sanctioned.

AAT background, doesn't pick up mystic or use 1000 starting xp, goes through first mission, then buys psyker. Sanctioned or not sanctioned?

Not sanctioned.

The fluff the character has in mind at the moment is that they're currently still just discovering their powers, that they underwent the training, yet they haven't developed them to the point where they're usable. Through their experiences with the Inquisition, they then finally get in touch with their mind and learn how to use it. I'm just trying to figure the best way to represent this without any kind of shenanigans regarding the witch hunters.

Someone correct me if I'm wrong, but the fact that someone takes AAT doesn't mean that the character was taken aboard a black ship, transported to Terra, underwent the sanctifying ritual and got the stamp "Imperium Approved".

If he's only discovering his powers now and they manifest themselves during a mission, it seems like he's going rogue. Now depending on what kind of inquisitor is owning his ass, this could mean that the acolyte is promptly put on the first black ship available, or it could mean that the psyker is too valuable for the inquisitor to lose for such a long time that he/she decides to protect him. Who better to deceive the Imperium than an inquisitor.

You might want to add another layer to this to answer the question why the inquisitor would take the risk (because not sending him over to the black ships seems to me as a transgression of Imperial law) to harbour a rogue psyker. I think it could be great fun to have some kind of prophecy in play where the inquisitor knows that the psyker will prove invaluable in defeating an enemy, but at the same time (because he/she lacks the sanctioning ritual), will also prove to be a danger to one of the other acolytes (or even him/herself). Seeing how psykers will sooner or later wreck havoc by suffering perils of the warp, this isn't even a prophecy that is completely unrealistic.

Even though the character becomes a rogue, I'd allow him to play it. There are just too many ways to use this storywise

According to the book, the AAT background most likely implies that the person is a survivor of the Black ships. The alternative is that they were a warden on board a black ship or within the organization on terra, or hunter of new latent psychics. It's definitely feasible that someone could come from one of those alternate backgrounds could actually be a latent themselves and not know. Ooh, there's potential, because they know EXACTLY what will happen if they're found out...well, they're gonna have a very good reason to hide what they are. Granted, I find it difficult to believe that they wouldn't have been detected, with such close proximity to those dedicated to finding new psykers.

If a psyker doesn't have a strong enough talent when he undergoes testing, he nourishes the Golden Throne/Astronomican or he is transformed in an astropath if he has what it takes.

I couldn't have been trained as a psyker without "usable" psychic powers and so, starting to be able to use his psychic powers later would mean that he wasn't trained with this potentiel before, and he is a threat.

But there could be a lot of way to do this: the AAT locked his powers for some unknown reason for them to be ready to use sometime, and now they are, or many other special stories that you could create. But basically, I would say this is not a sanctioned psyker.

I'm also okay with add007USA's take on the AAT background, in my opinon you could be a shrine world adeptus astra telepatica warrior, reprensenting one of the soliders guarding the AAT's operations.