> Thought experiment: you use nice pretty reflectors to smelt aluminium. You now have a ball (or, more likely, an expanding cloud) of +/- 700C molten metal.
Actually, extracting Aluminum is more complicated than just heating, since most of that metal everywhere (Earth and space) is in the form of oxide minerals. However Iron in the form of metallic asteroids *is* available already reduced to metal, so I will substitute that in my discussion. You build a rotating circular crucible and throw chunks of metallic asteroid into it. Focus enough sunlight on it to melt the batch. Bits of rocky inclusions will float to the "top" (center) because they are less dense, and the molten iron will sink to the "bottom" (rim). Throw in a bit of carbon from the C-type asteroids, since Iron + Carbon = steel. The bottom of your crucible has a hole that you tap to extrude the molten metal, which then passes through cooled rollers to provide a final shape. On Earth this is called "continuous casting". The rollers can form an "H" shape for structural beams, flat sheet, or whatever else you need, by just choosing roller positions. Cooling water goes through the rollers, and out to radiator pipes. They don't have to cool to room temperature, just enough to keep the rollers from deforming. Since the radiators will be rejecting heat at a pretty high temperature, they don't have to be very large.
> I'm not saying we should shitcan the whole idea, but the "Futurist" camp really has to stop talking about how trivial things are once we get most of the way out of the gravity well,
Actual space systems engineers like myself don't trivialize the tasks. Most space enthusiasts don't even know what materials are available to work with, or what the solar flux is, or the realities of working in the space environment. But some of us do know all that stuff, collectively. I don't know everything, either, and I work in the field. Generally you need teams of specialists in different subjects to complete a project. So you won't get a complete answer in a forum comment. You get it in a study report that lots of people contributed to.