Optically flat isn't even needed per se. You can weld metals by slipping two blocks a bit under pressure side by side just like that in standard atmosphere, and

Optically flat isn't even needed per se. You can weld metals by slipping two blocks a bit under pressure side by side just like that in standard atmosphere, and when there's some sort of low-pressure environment it just takes that much less of a slip. No need to go down to space-grade vacuum at all.