@Gimli, enjoy.

Thought you all would like this.

Found that on Youtube myself. I thought it was fascinating – especially the part about how primitive islanders with no tools could make a perfectly flat surface by rubbing three rocks together in sequence, and that this flat surface is the foundation for making perfectly straight rulers, circles, and so on upon which all of modern precision machining is built.

It is surprising that a mirror for a telescope can be ground without any precision tools. Without any tools at all.

Indeed, but a spherical shape for such mirrors, while most common, easiest and cost efficient, is not optimal. hence the term “spherical aberration”. Optimal shape (ceteris paribus) is a parabola, such that all incoming rays are reflected to the exact same point.

But nature is pretty good at making round objects…this is why pebbles, after centuries of tumbling along in river streams, weeks in a rock tumbler, etc. approach a spherical shape.