So Navis Primer is finally up for purchase on drivethru, so I started reading it.

"But such communications are far from conventional, transmitting not words, but notions, emotions, and impressions. Even the strongest of astrotelepathic signals is little more than ritual and might very well cost the sender and the recipient their very souls."

Lets set aside for a moment a question on whether or not you could have a galaxy spaning empire without some form of FTL communication. This view is actually completely conflicted by all the examples we see in both the novels and the printed text of the books where you have messages being sent by astropaths that's eventually decoded as plain text.

I don't think astropathy is that dangerous, but I can see it being…images and symbols.

Like, you as an astropath are trained with the thousands of ciphers and codes, which you turn into images and symbols, then launch through the Warp (never know who is listening). Another astropath, trained in the same ciphers and codes, translate it into plain text and then give it to their masters.

So, you actually can double cipher it. An inqusitor can give an astropath a message in a cipher, the astropath turns that into the warp-cipher, then launches it. Then the astropath on the other end takes that, decodes it into plain text (which is ciphered) and then the Inqusitor's friend translates it into plain plain text.

I mean, all astropaths are trained on Holy Terra, so it actually makes sense for them to have a "shared" telepathic language that isn't actual text.

Yeah, as I understand it, if you tried to send "The orks are attacking!" as a single thought, you'd be lucky if they understood you. But if you just send "ALPHA. ALPHA. ALPHA. OMEGA. THETA." as five seperate messages, then it's much more likely that they'll recieve and understand you, which is why most of the printed intra-system messages we see read like telegraphs.

Actually Astropaths DO deal in symbols and imagery, with metaphor and subtext being their ciphers. The plaintext you see in astropathic communications is simply what they have decoded the messages into so that we boring, normal humans can understand it.
From the fiction in The Outcast Dead which happens in and around the Astrotelepath's HQ on Terra during the Heresy, i can be seen that at least half if not more of an astropaths training actually comes from the ciphers and decoding of messages and not just the raw ability to send and recieve.

FTL communication is something that simply does not exist within the imperium. The closest there is to it are the high powered Librarians of the Astartes whose telepathic communication allows them to instantly communicate across entire sectors. Of course this doesn't benefit the Imperium at large but it explains how the Astartes can continue to be an elite, efficient strike force despite being a mere shadow of their former selfs and without so much support from Terra.

When role-playing an Astropath, when the captain told me to send a message, I would go into a trance pose and start mouthing everything he (and the person he's talking to) says in his message, saying the last few words in his sentence. For example:

Captain: "This is Captain Marcus Augustus of the Adeptus Terra. This is Imperial space. Identify yourself or be destroyed."

Me: "… or be destroyed."

This became hilarious when the Captain muttered something under his breath that he REALLY didn't want the other party to hear. Speaking with Eldar or Chaos sorcerers would cause convulsions, nosebleeds, and/or fainting.

Why would you be sending a message ending in "identify yourself of be destroyed" using astrotelepathy? My understanding is that such messages go to other Astropaths. Surely when using astrotelepathy you know who you are sending to. If you don't know that, how do you even know they have their own Astropath to receive the message?

Am I misunderstanding the whole concept here?

Well, you'd never use astropaths to communicate with anyone you can contact via the vox.

Zoombie said:

Unless you're worried that the vox signal is less secure than the cyphers of your Asropaths.

HappyDaze said:

Then use a whisper-link. You know, direct laser communications. You can only snoop on those if you are directly between the two points (and in space, that's pretty hard.)

An astropathic message has - like all psychic powers - a definite risk and uncertainty. I'd take a machine spirit over that any day of the week myself…

HappyDaze said:

But why would you encipher a message that ended "Identify yourself or be destroyed?" If you don't know who they are (and whether or not they have an astropath handy), why the hell would you assume they could decipher it, let alone receive it?

Plynkes said:

I wasn't tying my answer specifically to transmitting a threat, so I really don't have any answer for your question.

Zoombie said:

There's also the possibility that those you're trying to communicate with don't have vox. Not all xenos might have vox/radio technology (although most of the known ones seem to), but then you're still guessing if they can actually receive and understand astropathic sendings - especially across a language barrier.

HappyDaze said:

Ah, sorry, it's just that those were the specific circumstances we were discussing, so perhaps I can be forgiven for not realising your comment was referring to something else.

Plynkes said:

When I quote something and give a response to that quote, that's specifically what I'm addressing.

Plynkes said:

Much of the time, I surmise the sender of an astropathic signal chooses the place, and hopes for a receiver. With such vast distances, and time being so wanky thanks to the Earp, you have no idea where anyone ever is. Say you wanted to reach the Inquisitor Council on Scintilla. They have astropaths, but any particular one might have had an incident, and blown themselves up, been killed through the machinations of others, been called away for another reason, or whatever. You really don't have a good way of knowing which Astropaths are in the Tricorn, at the time, and who will still be, when your message finally gets there. I believe you send to a location, skipping it off the heads of other Astropaths, as it goes, to maintain integrity, and it eventually gets to Scintilla, to the Tricorn, and to an Astropath of the Council, who decodes it, and takes it to the Council, or the Lord Sector, or whoever needed to know what's up.

Something I'm sometimes confused by, when the Horus Heresy happened, half of almost everything fell to the Ruinous Powers. They didn't JUST get way too many Space Marines, they got what became the dark Mechanicus, whole chunks of the Imperial Army, and many of the ships and vehicles that came with them. Since then, numerous other individuals have also succumbed to the taint of Chaos. How does Chaos have no "astropaths" on their side, intercepting the messages, who know the same ciphers? I'm assuming it's the Soul-Binding, in a more RP version than the as written mechanic, but I don't know, and with everything else Chaos also has, it's kind of odd that the one thing they didn't pick up was Astropaths, to listen in on the Imperium's messages.

Well, almost 10 milenia between the Horus Heresy and the present day (and the fact that a non-negligible proportion of assests went traitorous during the Heresy) makes it probable that the ciphers themselves have changed, if not regularly, than at least after or during the Heresy (after all, what is the point of ciphering things if anybody who wants to can decipher them, which, after 10 000 years of using the same one in a galactic empire, could be pretty much any one with any psychic powers). This is without considering the importance of using different ciphers to stop specific individuals from decoding them (such as a rival's inquisitor or an eldar Farseer).

As for more psykers falling to Chaos, the soul-binding and conditionning of trained psykers means that only a very small proportion of lost psykers actually turn to Chaos, and Chaos, due to its own nature, is usually not a centralized enough power to maximize use of it's assets.