Bau eines Teleskops:
Why not simply buy new if it isn't really more expensive than making your own ?
Several reasons (which are not necessarily true for everyone) :
Why not automate the process of grinding and polishing the mirror with a machine ?
This is possible indeed, and this is how commercial mirrors are made. However, machines have (in this context) a major inconvenient : they repeat the exact same action and potentially also the exact same error (well, they are man-made...). Human hands however will never do exactly twice the same movement, thereby adding a random factor in the polishing phase. For once, this is interesting and can in fact lead to very high precision.
What's the difference between the abrasives called Carbo and Corundum ?
Corundum is a natural crystalline form of aluminium oxide (Al2O3). Carbo (full name Carborundum) is a synthetic mineral, formula SiC (Silicon Carbide). Both minerals have a high hardness, but corundum leaves fewer pits than carbo. 2O 3)