re: #66 zombie

[snip] Once you have that, you can then make parabolas of any curvature.

With those, easy enough to make parabolic mirrors. [snip]

A typical cheap mirror-type telescope will use a spherical mirror with a focal length about 10 times its diameter. F/10 in camera jargon. A small F/10 spherical mirror is such a close approximation to the ideal F/10 parabolic mirror that it is already near-perfect, optically.

A spherical surface is the easiest to fashion with glass or metal, because two spherical surfaces of the same curvature can remain in intimate contact as they slide over each other. This is not true for any other shape surface, except flat.