I wasn't sure you could Teleport to a location in mid air or in space. But let's say you could, and just run with it. You place your platform at the hight of Lunar orbit, whether there's a moon there or not. Take your boulder and work from there. How did you accelerate the whole rig to the speed needed to maintain orbit? Teleport won't do that, and there isn't a spell in the game that will give you that kind of speed. That's 3,600 kph, which is 2,160 mph, which is a base move of 19,000.06 feet per round (give or take a little). And you have no way to really gauge your speed, so the whole rig may just come crashing down in a fiery streak, or spin off into too-wide an orbit and throw all your targeting and glass grinding calculations into the trash. (Aren't orbital mechanics fun?
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As far as your mirror making is concerned: Considering that the precision needed is greater than anything we can make today, I'm not at all sure a +65 Craft check comes close. Even making "flat" mirrors to use for segmented focusing would have the same issue, since precise flat and precise concave/parabolic face many of the same problems.
So let's pretend you made a mirror array the size of Delaware. That would let you double the amount of sunlight hitting an area the size of Delaware. Not exactly a laser death ray.
Now, you get to make a few thousand individual attack rolls, all at once, one per mirror. This depends on your servants (be they Unseen, Warforged or Undead) each rolling a natural 20 to focus them all on an area the size of Delaware. This actually plays in your favor though, since it removes any need for them to have any weapon proficiency in mirror array. Nothing less than a natural 20 will do, and you can't aid them all since they all have to work at once to hit the moving target. By the time you get one set right and go on to the next, the target has moved away from the first focal point.
But here it gets easier. The size mods in the game stop at a certain point, so hitting a castle is no harder than hitting a county, or an area the size of Delaware. And this works in your favor, since you probably want to hit the castle, not the county.
Range mods are a different matter. For most weapons the maximum range is listed at 5x the range increment, so a Composite Longbow tops out at 550 feet (5 times the 110 foot range increment).
But let's pretend that that isn't a problem or a limit, and let's give your mirror array a range increment of 100 miles. Overly generous, I know, but I'm a soft hearted kind of guy.
So you're shooting at a target, with something like +16 to hit from size, and -40,000 to hit from range (give or take a few thousand.)
Did we mention that the target is on a surface moving past at 1,000 mph due to surface rotation, complicated by the orbital motion of the moon? But that's okay, because game rules don't take target motion into account.
Now, the next thing to consider is this: Do you really want to drag physics into a magical role playing game, when you start seeing problems like this? And I'm just getting started.
