Fair enough.
As a DM, you can create/approve any type of world feature you like, and when people ask for an explanation, you (as DM) can simply answer, "You don't know how they built such a thing."
But I'll cut the obstructionist BS.
Spotting the planet wouldn't be a problem. Spotting a particular building would be. The only man made structure visible from orbit to the naked eye is the Great Wall of China. Even with magical aid, it would be a problem to see and identify a specific castle, or even a city. Some sort of divine guidance could bypass that, of course. In fact, you could Gate in a Solar to aim it, and he/she could Contact Higher Plane for divine aid. You'll need an army of Gnome craftsmen to build the aiming mechanism for all the mirrors at once, but that's just backstory.
Hitting that building would also be a problem. You, as DM, can create any custom aiming ritual the "bad guy" needs, of course, and make it as long and drawn out as you like for dramatic purposes.
Or maybe that's part of the puzzle the PCs have to figure out: How is the BBG managing to hit, precisely, whatever castle he threatens, first shot every time? Is there some magical equivalent of a homing device there? Is it being fired through a Ring Gate, so he need only hit the nearby Ring Gate while the other half is hidden someplace near the castle, aimed at it? Is he using some kind of divine or fiendish adviser that can be killed, dissuaded or subverted?
Or is the challenge finding a way to his mountain stronghold, hidden someplace within the darkness between the stars?
Lots of good story potential there.
I have no issue with you, as a DM, doing anything you want with the idea.
But I thought you were asking as a player, and I replied to you in that context. Sorry for the misunderstanding