Stop the cannon is a mission in The Precursor Legacy that Jak and Daxter completed to earn a power cell. According to Samos Hagai, a cannon at Misty Island had been bombarding the dark eco silo, until Jak managed to stop it. Apparently, Samos could "see them bombarding the Precursor silo" from his lookout tower.
Walkthrough[]
First of all you need to enter the Lurker ship and get on the stern. Here get on the long wooden walkway above the lake. This can be done right away after the mission Climb the lurker ship. There's two types of logs coming down here, a normal rolling one which can be avoided without much effort (jump over it) and a bouncing one.
The bouncing log is particularly annoying, although it shouldn't be hard to avoid if you roll (assuming you correctly guessed the pattern) right when it is about to hit you. They come down in a set pattern, first two rolling logs, followed by a bouncing one where after it is repeated.
Keep moving forward, using any of the side platforms to rest if necessary. Eventually you'll hit the top and you can jump off. Move left here, defeat the two babaks and the power cell is yours.