No...you need to edit the four entity files (AbilityTechOrbitalCannon, AbilityTechOrbitalCannonLoyalist, AbilityPsiOrbitalCannon, AbilityPhaseOrbitalCannon) associated with the cannon abilities...
You need to change this Boolean
isAutoCastOnByDefault TRUE
to FALSE
Of course, in order to edit these files, you will need to know how to convert entity files from BIN format to TXT format....and then you will need to know how to set up a mod (where it goes, what to name your folders, etc.)...there are guides for both of those items...