Parabolic reflectors for solar applications are nice ... After you´re done with this, you end up with a nice flat mirror. Here´s where the vacuum is needed to force the film into parabolic ...

To me, this means a large parabolic reflector ... For a visible light telescope at a wavelength of 400 nanometers, this means a parabolic mirror needs to have a surface finish that is perfect ...

This was the first working reflecting telescope. Other designers proved unable to grind mirrors with a regular curvature, so the reflector remained largely a curiosity until the mid-18th century ...