Spoiler
You purchase the death ray laser from an NPC in Freeside
So the actual gun for the death ray is in a town I haven't heard of up till that point, from an otherwise unremarkable NPC...
SUGGESTION
If a player choose "ARCHIMEDES II" and activates the solar array, the quest should not complete. It should continue one step further to "Obtain the trigger for the Archimedes II platform", with a map marker on the NPC who has it.
Of course, this could probably be done as a mod... but anyone who downloads the mod doesn't need it because they already know where to go.