Hello Commanders!
Fixed weapons without a target actually have *no* convergence - they fire straight forward parallel to the centre line of the ship.
When you have a target, fixed weapons have a very limited amount of aim assist to angle towards the target.
This allows the weapon to converge on the target, but, depending on weapon placement, it can create a minimum convergence distance, or "blind spot". Targets closer than this distance will cause the aim assist to "lock" at its maximum angle before the weapon is pointed directly at the target.
Such minimum convergence ranges are determined by how distant a hardpoint is placed from the ship's centre line (assuming you are directly facing your target!)
For example, both of the Sidewinder's small hardpoints are very centrally placed so the aim assist only has to change the weapon's direction by a tiny amount to nail the target. A ship needs to get right up in your grill before the angle between the target point and the weapon is greater than the aim assist can cover (and normally even when this happens the attack will still part of the ship - just not the exact target spot).
On the other hand, the hardpoints on an Imperial Clipper's nacelles are very far from the ship's centre line, creating a large blind spot where the target is outside of the arc that the weapon's aim assist can rotate to (small ships can literally sit between fixed weapons on these hardpoints and have both attacks pass harmlessly by on either side!)
Each weapon on the targeting HUD has a dot marker that shows the current trajectory of attacks made. Think of the dot as the end point of a line drawn from the weapon barrel. When you have nothing targeted line stops 500m from your ship.
When you have a target, the line stops at the distance the target is from your ship. If the target is beyond the minimum convergence distance the dot will be able centre on the target. As the target gets closer and passes into the weapon's blind spot, the dot marker will appear to move away from the centre of the target; actually it's the target moving away from the imaginary line drawn from the weapon barrel, which is locked at maximum rotation.