Everything else potentially carried by an Imperial Navy vessel is already covered, so I thought I'd try my hand at providing rules for the other weapon seen mounted beneath the armoured prows of Imperial Navy Cruisers...
"A Nova Cannon is a huge weapon, normally mounted in the prow of a ship so that the recoil it generates can be compensated for by the ship's engines. It fires a projectile at incredible velocity, using graviometric impellers to accelerate it to close to light speed. The projectile implodes at a preset distance after firing, unleashing a force more potent than a dozen plasma bombs."
To fire a Nova Cannon, nominate a point anywhere between 4 and 18 VU within the ship's prow arc. The weapon generates a blast wave several thousand kilometres across, and may catch several ships within its effect if carefully aimed. Nova Cannons are extremely powerful weapons, easily able to wreak vast destruction upon enemy vessels at great range, but are inaccurate. A Nova Cannon may not fire if any Manoeuvre Actions have been performed which required a Pilot (Starship) test with a difficulty of worse than Challenging (+0), or if the ship is Crippled.
Roll to hit, applying a modifier based on the distance to the target point. If the target is within 6 VU, you take a -10 penalty. If the target is between 6 and 9 VU away, you take a -20 penalty. If the target is more than 9 VU away, you take a -30 penalty.
A successful roll scores a single hit against every ship within half a Void Unit of the target point (in most cases, ships will not be close enough together to hit more than one at a time). If three or more degrees of success were scored, a direct hit has occurred, and a single one of those ships suffers 1d5 hits instead. Hits from a Nova Cannon deal 2d10+4 damage ignoring armour. Void Shields apply normally to hits caused by a Nova Cannon. The Nova Cannon has a Crit Rating of 5, and will only deal a single Critical Hit at most, no matter how many ships are struck (the ship hit multiple times will suffer the critical hit)
A Nova Cannon may only be fitted onto a Cruiser or larger vessel with an Armoured Prow - the massive superstructure of the Imperial Navy's signature prows are perfectly suited to withstanding the heavy recoil of a Nova Cannon firing. It uses 15 power and takes up 10 space, including a dedicated magazine (which will, if the Nova Cannon is damaged, explode in the same manner as a Munitorium, described on page 205 of the Rogue Trader rulebook). The entire Nova Cannon costs 5 SP.