To clear a level or sector, you have to destroy 15 alien ships (there’s a counter in the upper left). At the end of the level, some kind of mother ship (called a sentinel) floats around in the back protected by so-called green blockers. It can only be destroyed with a torpedo (you have 3 per level). To fire a torpedo, you have to push the joystick up (make sure that when you fire, there’s nothing between you and the sentinel since a torpedo destroys the first object it encounters and that’s it). The yellow thingy (called a rejuvenator) is not supposed to be shot at since it gives you an extra ship. There are supposed to be 99 sectors. Wow! That’s a lot.
As you can see in the video, I was not able to get the sentinel ship but that was a good try, I think.