Aluminum is indeed a very abundant metal in the Earth's crust. Whether it's the same in the D&D world is an interesting question, but actually irrelevant.
You can't smelt aluminum the way you do most other metals. That is, bauxite (aluminum ore) doesn't give up its oxygen when heated, it remains aluminum oxide. To get it to give up its oxygen and become a workable metal, you need electricity, and not just a momentary jolt. You need lots and lots of electrical current over an extended period of time.
There aren't any electrical generating stations in the D&D world, at least not that I'm aware of.
All it would take would be the knowledge of aluminum's existence to prod some wizard with a smithing fetish to devise a spell to get the job done. I personally don't think that is any form of a stretch.
Regarding segmented mirrors: Binding mirror segments with force effects or Immovable Rods sounds like a wonderful idea. How were you planning to aim the thing, if it can't be moved?
I was just suggesting a way to bind them together, just because the way I suggested won't work doesn't mean that binding them together wont work. ^^
Regarding focusing an array of mirrors onto a single lens: You send a thousand beams of reflected light into a lens, and you get a thousand beams of light coming out. The lens doesn't convert them into a coherent beam.
That doesn't change the fact that there is still x amount of light energy all going to the same place, the majority of such being converted into heat energy when coming into contact with the earths surface or anything that absorbs light. Thus, a massive scale disintegrate is born.
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.
Isn't it a pretty huge assumption to say someone must SEE something in order to aim at it. Knowing somethings location, in degrees, fractions of degrees, or something else, in relation to several visible landmarks would make "seeing" your target completely irrelevant to aiming at it, from an orbital perspective. You think astronauts can "see" where they are going when they come in for a landing? It is all just calculations and fore-knowledge as to the location of things.
Question: Can you see through one Ring Gate and out the other? They transport *objects*, not visible light or attacks spells.
Energy and Mass are the same thing, simply in different form. To say something like a Ring Gate would not function in a similar manner to a blackhole (effects everything, even gravity and time), is illogical. If it transports anything, it transports everything, unless specifically stated otherwise. In my opinion, saying it transports "objects" and nothing else should not be assumed to mean one can't see through it. I however don't know the spell, so am just making arguments of logic. XD I think the limit of 100 miles does a good job of keeping people from using it to suck away earths atmosphere, or cover earths surface in magma.
@Greefield My god, you're fun to talk to. <3!!