I read somewhere once that Sanctioned Psykers undergo a different series of processes than an Astropath (taken to Holy Terra, soul-bound to the Emperor by viewing the Golden Throne) but don't know the specifics. Can someone fill me in? Everything beyond "holy hell I'm on a blackship surrounded by screaming witches and Sisters of Silence" is a mystery to me.
Very little is known for certain - indeed, the only concrete sources regarding the Silent Sisterhood are ones describing the Heresy-era Imperium, so we don't actually have any evidence that they still exist… an issue further compounded by the fact that the psychic cargo of a Blackship either end up dead or mind-wiped (more on that in a moment).
As I see it, the key to getting a handle on the whole issue of psykers in the Imperium is understanding the reasons why and the scale of things. I'll aim for brevity, but the subject is somewhat big so this might take a while.
The Imperium is vast - it is the single largest endeavour in human history, encompassing more worlds and more lives than any human civilisation has ever achieved. Nothing humanity attempts will ever be as large, for the Imperium is the result of the vision and drive of a being beyond mere humanity, and the Imperium as it exists at the close of the 41st Millennium is naught but a pale reflection of what the Emperor desired, the remnants of a shattered dream pieced together by the feeble comprehension of lesser men.
Galaxy-spanning civilisations do not easily exist, for the size of the galaxy is a virtually insurmountable problem to overcome. By the use of Navigators to guide vessels through the Warp, and Astropaths to send messages over vast distances, is the Imperium maintained. But psykers are also a peril, and the unguarded mind of a nascent witch or wyrd is a dangerous thing with the potential to inflict cataclysmic harm through the influence of daemons and warp-spawned monstrosities like Enslavers and Psychneuein.
So it is that every world (less those ruled by the Adeptus Mechanicus and Adeptus Astartes) must give up every psyker to the Blackships, for the good of all mankind. The Blackships are numerous, advanced and guided by Navigators serving under ancient bonds of service, and each plies the galaxy in a great circuit, visiting worlds, collecting Psykers and eventually returning to Terra. A given Blackship may visit a world maybe once a century, but there are enough Blackships to ensure that no one world is left unvisited for too long.
Of the psykers collected, some will be too dangerous to live, and will be executed on the spot. The overwhelming majority are meagre of power and feeble of will, useful only as fodder to sustain the Golden Throne. These are collectively referred to as Sacrifices. A minority are sufficiently strong to be valuable, and will live out their remaining lives in service to the Imperium.
Some of this determination - a prolonged series of agonising tests - will be done aboard a Blackship. The rest takes place on Terra itself. While the Sacrifices are dragged off to serve in death, the useful psykers have their horrifying memories of the Blackship wiped from their minds and they are then divided into three categories. The majority are Secondary Psykers - strong enough to serve, but too weak of will to defend themselves from the horrors of the Warp. Through sheer necessity, these are taken before the Emperor and Soul Bound, their eyes burned out by the Emperor's Light, at which point they begin their training and service as Astropaths of the Adeptus Astra Telepathica, sending messages across the void in service to the Imperium. The second category are chosen at random to join the Choir of the Astronomicon, giving their lives slowly over months or years to fuel the beacon that guides Navigators
A few, however, are sufficiently strong-willed to defend themselves from the Warp's influence that they do not need to be Soul Bound - these are Primary Psykers (the terms Primary and Primaris used to be synonymous, though the term Primaris is now used to specifically indicate particularly skilled/powerful/experienced psykers). These rare individuals, after prolonged and extensive testing, are Sanctioned for use by the Imperium in a variety of ways by a variety of organisations, their training and deployment overseen by the Scholastica Psykana.
Becoming a Sanctioned Psyker is essentially being proven to be sufficiently powerful and stable to serve without Soul Binding. Arguably, Sanctioning does not need to occur on Terra, but as the overwhelming majority of all psykers in the Imperium are being taken there anyway, it's easiest just to skim the Primary Psykers off the top when a Blackship reaches Terra and finish the Sanctioning there.