Using the T20 rules you can add another Sphere Affinity called Astropath which has a DC of 1.  The Astropath Sphere has the following disciplines.  The major disadvantage of gaining this discipline is the requirement to be marked by the Emperor.  This mark blinds the telepath but also bestows the feat Blind Fighting.  See the Adeptus Astra Telepathica for more information.

If you have access to the Psionic Handbook I recommend discarding the T20 psionic system and using  the system listed in that handbook as it suites w40k universe better.  The following entries have assumed you are only using T20 Psionics. 

To modify them for Psionic Handbook compatibility add a new discipline called Astrotelepath.  The key attribute for this discipline is  Intelligence.  Each Rank listed below is the equivalent Psionic Power Level.  Cost becomes Power Point cost for the power.

For more information on Psionic powers in the w40k Universe see the background information listed after the Astrotelepathy discipline.

ASTROTELEPATHY                                                                                                                      Sphere: Astro-telepathy                                                                                                                     Rank: 1                                                                                                                                          Cost: 1 per message                                                                                                                      Duration: Instant                                                                                                                               Benefits:  This is an extremely long-range telepathy that permits Astropaths to receive and transmit messages over distances up to 50 thousand light years. The use of this ability is successful only 50% of the time. Messages are often distorted or lost.

LOCATE WARP GATE                                                                                                              Sphere: Astro-telepathy                                                                                                                     Rank: 2                                                                                                                                          Cost: 2                                                                                                                                    Duration: Instant                                                                                                                               Benefits: This ability is only useful in warp-space. Within warp space this ability can be used to locate warp portals through to real space 50% of the time. 

PSYCHIC BEACON                                                                                                                         Sphere: Astro-telepathy                                                                                                                     Rank: 3                                                                                                                                          Cost: 3                                                                                                                                    Duration: 1 hour                                                                                                                               Benefits: An Astropath with this ability is able to broadcast a psychic beacon similar to the Astronomican but far less powerful. The range of the signal is a 10 light year radius around the position of the Astropath. Within this sphere of space an Astropath can guide a ship even without access to the Astronomican or computers, beyond the galaxy for example. The ability cannot be used from inside warp space, so an Astropath on board a spacecraft is unable to provide a signal (although if two ships were making alternate jumps it would be possible for each to derive coordinates from the other).

WARP SPACE TRAIL                                                                                                                   Sphere: Astro-telepathy                                                                                                                     Rank: 4                                                                                                                                          Cost: 4                                                                                                                                    Duration: 1d10 days                                                                                                                              Benefits:  This ability allows an Astropath to place a psychic homer into the mind of a member of a spacecraft crew. It doesn't matter which crewman is affected, the victim wouldn't even be aware of his predicament. The homer can only be placed from a short distance away (up to 1000 kilometres in clear space, but as little as 10 kilometres in a busy spaceport). Once placed the homer lasts for 5-10 days and its signal can be detected within 1000 kilometres by the Astropath. If during this time the ship makes a warp jump the Astropath will be able to sense its destination. Of course, to successfully trail a ship through warp space the trailing craft must remain within 1000 kilometres in real space, otherwise the signal will be too weak to follow.

WARPSPACE AND PSYKERS 

The material universe is but one aspect of reality. There is a quite separate and co-existing immaterial universe. This is commonly known as the warp or warpspace, although it is also known as Chaos, the otherworld, the ether, the empyrean, the void and the immaterium. Warpspace may be explained in terms of an endlessly broad and deep sea of raw energy. This energy carries within it the random thoughts, unfettered emotions, memory fragments and unshakeable beliefs of those who live in the material universe — it is the collective mind of the universe itself.

All living creatures exist in warpspace as well as in the material universe, although most are not conscious of the fact. Just as a man's body inhabits the material universe, his soul inhabits that of the warp. The body is part of the universe and made of matter: the soul is part of the warp and is made from the stuff of raw Chaos. Human sensitivity to the warp is not generally well developed. The soul itself is not aware: it is simply a coherent lump of Chaos energy maintained whole by its anchor to the material body. However, in a minority of people this sensitivity is far more finely tuned. These people are psykers and they are able to consciously control and use the energy of the warp to affect the material universe. There are many kinds of psykers, not all of them Human, some of whom are tolerated or encouraged within Human society while others are regarded as dangerous and are actively persecuted and destroyed.

PSYCHIC POWERS

As power from the warp flows into realspace, it splits into eight parts, each perceived by those with psychic awareness (often called the second sight) as a separate colour. Just as the warp comprises tides and currents of emotion that over the millennia have melded together to form the great Powers of Chaos, so in realspace each of the colours of psychic power draws on a certain type of emotion or energy from warpspace. This gives each colour its own distinctive effects when used by a psyker. A small amount of raw energy from the warp leaks through into realspace all the time. Those with psychic powers see this energy as layered mists of colour, building into boiling, turbulent clouds and multihued storms where the barrier between the warp and realspace is particularly tenuous.

To cast a psychic attack, the psyker pulls energy of one colour into himself, draining the surrounding area as he concentrates and focuses the colour. When he uses his powers, he further weakens the distinction between warpspace and realspace, allowing more power to flow through, providing additional impetus to his attack and replenishing the mist of colour that surrounds him. To those with psychic second sight, it appears that the psyker is the centre of a maelstrom as fragments of colour whip and twist around him. As he gathers all the energy of one colour, the storm dies for a brief moment and he stands at the calm centre of the boiling clouds — he moulds the power into his chosen form, turning it with his mind from raw energy into potent weapon of attack or a shield of defence. Then he releases the pent-up energy, hurling it at his enemy or pushing it out to form an impenetrable shell around him.

Those with the second sight see psychic attacks in many ways. Each psyker interprets what he sees according to his understanding of the warp: some as a dance of pure colour; others as a strange geometry of mystical symbols drawn from the arcana; many as images of power and destruction taken from the mythologies of their homeworld. Where one may see bolts of startling colour, others will see the talons of huge beasts grappling with their enemies or strange forms with a Daemonic glint in their eyes ripping and tearing. For some, skeletal hands reach out of the darkness with the touch of death where others see all-consuming hellish fires burning with the souls of the damned. One will see spiders and beetles whose eyes glow with an uncanny radiance and whose feet send out sparks as they skitter along faint webs of colour to reach with thin feelers into the minds of their victims. Another will see pools of darkness that spread from the caster to consume everything they meet with the insatiable hunger of the warp.

Even those without the second sight see flickering shadows, or catch a sight of something from the corner of their eyes, bringing a moment's dread apprehension before the psyker's attack bursts upon them with its full energy, surrounding them with deadly fires or a hail of coloured bolts that strike down all in their path.

As well as using the power of the warp by drawing it into realspace, the psyker can reach out within the warp itself to clutch the soul of an enemy and break the thread that connects it to a living body. For a second the eyes of his victim will go blank and any who look into them will feel drawn in, sucked by the black emptiness of death, before the victim crumples to the ground in silence — only his soul, torn apart by the psyker, screams unheard with the agonies of annihilation. The links of those who are weak are easily broken; the psychically strong are anchored to their souls with adamantine chains and only the most powerful of enemies can threaten them. As they feel the presence of animosity in the warp, they can concentrate their power into the chain, setting it aglow as if just pulled from the fires of a furnace, throwing back the assailant with a white psychic heat that bums any who approach.

PSYKERS IN THE IMPERIUM

Most Humans do not have psychic powers, although all Humans have at least a limited potential for psychic activity. However, a small but growing minority of Humans do develop tangible powers. These people are called psykers by the Imperial authorities — on their own worlds they may be known by many names: warlock, witch, necromancer, spirit walker, exorcist, speaker in tongues, shaman. Psykers are dangerous individuals whose powers can only be tolerated when safely harnessed within the Imperial organisation: the psychic universe is the universe of Chaos and therefore perilous. It is a universe inhabited by Daemonic aliens that care nothing for living creatures and wish only to use and destroy Humanity. All psykers, even the most powerful, offer these aliens a potential means of entering and affecting the material world.

Every planet in the Imperium is bound by law to control its psychic population. Persecutions or witch-hunts are an everyday part of life on most worlds. The same laws oblige rulers to set aside a levy of young and relatively promising psykers for transport to Earth by the Adeptus Astra Telepathica.

The Adeptus Astra Telepathica

The Adeptus Astra Telepathica is dedicated to the recruitment and training of psykers for service throughout the Imperium. The headquarters of the organisation is on Earth, but its ships travel the Imperium and its offices extend over most of Human space. The institution is divided into a teaching body called the Scholastia Psykana and a recruiting body known as The League of Blackships. These two are united under the Master of the Adeptus Astra Telepathica and his advisory council of several hundred senior officials drawn from the main divisions.

The League of Blackships

The League consists of a substantial fleet based throughout the Imperium. The ships visit each world every hundred years or so. As the fleets approach their destination, the ruling Imperial Commander is instructed to prepare the customary levy. On many planets, this is a holy time — a time for rejoicing as the young hopefuls gather for their chance to be taken to the stars and serve the unseen Master who rules them all. Once the levy has been collected, the Blackship Captains make an initial evaluation of their cargo before proceeding to the next world in their circuit. When the holds are full, the Blackships turn towards Earth. It is common for Inquisitors to travel on board these ships, as this gives them a good opportunity to investigate a planet's potential for psychic corruption and other heresy.

The Scholastia Psykana

The Scholastia Psykana is a vast teaching institution dedicated to the training of psychics. Most recruits are drawn from the levy collected by the Blackships, but a minority are handed over by the Inquisition, the Judges or through other channels. The role of this institution is to teach young psychics how to develop and control their powers. The future of each psyker depends on his abilities and character. Initial evaluation divides the levy into several groups depending on their innate psychic power and their willingness to serve the Emperor.

The Chosen

Those whose powers and strength of character are sufficient to resist possession and Daemonic taint under normal circumstances are chosen to serve in an elite capacity. They are often known as primary psykers or the Chosen and they will learn to serve the Imperium in many ways throughout the galaxy. The very young may be indoctrinated into the Space Marines as Librarians; the most talented may become Inquisitors or Grey Knights. Even these chosen psykers are not invulnerable to the powers of Daemons and psychic aggressors, but their training gives them a fighting chance against all but the most potent of these creatures.

Astropaths

Astropaths are selected from the second ranking of psykers, those whose powers are considerable but inadequate to resist the dangers of possession or Daemonic corruption. Astropaths undergo basic training coupled with a thorough study of telepathy. They are taught how to use the Emperor's Tarot, how to cast horoscopes, and the practices of cheiromancy and augury of all kinds. Once they have been prepared in this way they undergo the unique Binding Ritual which gives them a little of the Emperor's strength.

Sacrifices

The psychic levy inevitably includes many whose powers are too random and whose minds are too vulnerable. If left unrestrained they would soon perish and their doom would lead to further deaths — maybe even to the destruction of entire Human worlds. In a teeming universe their loss is of no great matter but even in death they can serve, for the Emperor must feed upon raw psychic energy if he is to survive as the protector of Humanity. They become sacrifices to the Emperor, their souls leached from their bodies to sustain the Father of Mankind.

LIBRARIANS

A Marine chapter's Librarium is both its command and communications centre, and the repository for centuries of wisdom and history, culled from the reports, treatises and memoirs of the chapter's greatest warriors and finest minds. Because interstellar communications are achieved by psychic means, sending messages through the warp rather than through realspace, most of the Marines who work in the Librarium are psykers. Known as Librarians, they are recruited from among the youngest and most promising primary psykers trained by the Adeptus Astra Telepathica. Librarians also have potent psychic powers which they use on the battlefield to augment the conventional weaponry of their brother Marines.

Librarians hold a functionary rank, describing their role as well as their position. The four battlefield ranks of Librarian, in ascending order of importance, are Lexicanian, Codicier, Epistolary and Chief Librarian. Each of these enters battle ready to support his brothers with a range of psychic attacks and defences. They are fielded in many different situations, especially when Marines of the chapter face psychic opponents such as covens containing Rogue Psykers or followers of the Chaos Powers.

In addition to their duties as warriors, each performs an auxiliary role. The Lexicanian prepares a report of each battle for the chapter's records. The Codicier evaluates the reports of Lexicanians and provides a strategic overview of campaigns. The Epistolary is a more powerful Librarian — his role is that of chief psychic communications officer. He transmits and receives psychic messages on the battlefield.

The most important of all fighting Librarians are the Chief Librarians, superior in rank and psychic power. They report directly to the chapter's commander and hold overall responsibility for the maintenance of communication lines. They also scrutinize their subordinates' battle reports and give recommendations for honor awards, drawing on their intimate knowledge of the chapter's glorious past to appraise a Marine's valorous actions.

HYBRID PSYKERS

All Genestealers are psychic. They share a limited form of telepathy that lets the members of a brood communicate. They also use their power to hypnotize their victims before striking to implant their seed, lulling them into submission and then erasing the event from their minds. Only when the much-loved firstborn of such a victim arrives does the truth become apparent — by then the victim is ensnared, bonded to his Hybrid child by chains of parental love, strengthened by the pull of the Hybrid's latent psychic power. From this beginning, a brood of Hybrids and followers will grow, owing allegiance to their founder and Patriarch.

The psychic power of a brood is shared between all the members and is focused on the Patriarch. The Patriarch has the strongest soul and the greatest ability to draw power from the warp. He is a mighty psyker and his attacks can be devastating — to those unprepared to face him, it may seem as if the very gates of hell have opened when he unleashes his full power. He also passes on part of this power to his children, feeding and directing currents of warp energy towards them and strengthening their own psychic abilities. In Purestrain Stealers, this psychic power is latent. In many cases, however, Hybrids of the 3rd and 4th generation are able to direct their psychic power, casting psychic attacks to annihilate their enemies or shielding themselves and their kin from harm. Like other Stealers, they draw their power from the brood's shared presence within the warp; each Hybrid benefits from this presence, having a greater ability as part of the brood than he would alone. Because they draw power from the brood's strongly-bonded souls, the abilities of Hybrid psykers vary less than those of their Human counterparts. Even so, some Hybrids are gifted with much greater talents than others — it is one of these who will rise to the exalted position of Magus, standing at the right hand of the Patriarch, second only to him in power, and interpreting his will to the Human and other cult followers of the brood.