When Do You Get To Build Star Destroyers In Star Wars: Empire At War?

In the Imperial Campaign when do get to start building Star Destroyers or the version you see on the movie? I have got to the point in the campaign were you find out who the spy is and your orders is to hunt him down. I captured 4 planets that are capable of building the the Star Destroyers. And all of my space stations are fully upgraded. How much further into the game to go to build them?