Primary mirrorA primary mirror is also a form of distributed data management on the Internet.
For most of astronomy's history, primary mirrors used to be monolithic blocks of glass or other material, curved to exact shapes and coated with a reflective layer. This worked well, but as telescope diameters began to increase, the primary mirror became also the primary limitation on the telescope size: the mirror had to sustain its own weight and not deform under gravity. The limit was soon reached with the 5-meter Mount Palomar observatory and a 6-meter in the USSR. For decades, telescope sizes did not increase significantly.
Then, some new technologies were introduced: starting with the MMT, primary mirrors were constructed from small segments, merged (by physical contact or later by optics) into one large primary mirror. While the MMT was a 4.5-meter, the Keck telescopes used a 10-meter segmented mirror, and many others are in development.
Secondly, a thin mirror technology was used together with active optics: a very thin mirror (in the order of centimeters) is suspended by actuators in its optimal shape, against the force of gravity. This allows large non-segmented mirrors. This technique is used on the VLT and LBT, and in many other operating or planned telescopes.