one of the few technological fields where "bigger is better" is that of telescopic instruments used in the exploration of outer space. The resolution of a telescope for light capturing increases in direct proportion to the diameter of its primary mirror or lens. Prior to dedication of the first reflector-type telescope in 1917, astronomers had been limited to using refractive lens-based instruments. After the conversion to reflection, and even until only recently, the reflective mirrors were also produced from glass. The low thermal inertia of the larger glass lenses had a tendency to distort the incoming light because the air immediately adjacent the lens was almost always either hotter or cooler than the lens itself. In subsequent developments, the lens mass was reduced by sandwiching a glass core between thin outer glass plates, but this too had size as well as production limitations.
In the late 1970's, building of larger parabolic reflectors became more practical due to the advent of the computer. The mirror was made of segments like a jigsaw puzzle, which enabled the individual pieces to be made in the form of panels which are thin and light in weight. When assembled, the panels were individually tipped, tilted and pistoned up and down at balljoint-supported corners or sides under very accurate computer control. Under this technique, known as adaptive optics, each panel was kept within a 0.001 micron optical tolerance to the next adjacent panel. This was made possible by development of sensors which were able to detect even the minutest displacements. Constant adjustment, up to a thousand times per second, is made to compensate for the up and down tilt and rotation of the reflector and the attendant effect of gravity resulting from those movements. Wind, is yet another factor requiring frequent segment adjustment. One scientific article has described a computerized model of the effects of wind on such a segmented mirror as resembling "nothing so much as a manta ray thrashing in a turbulent sea".
Computerization has also led to effectively combining the readings from multiple smaller mirrors to achieve the resolution of a much large mirror. One proposed design is said to be able to combine six 3-foot mirrors with one 6-foot mirror to simulate the resolution of a single large 20-foot diameter mirror. While this technique has been known since the 1930's, it has become feasible only recently because of the capabilities of high speed computers.
A relatively recent and innovative panel design employs the use of pure aluminum, machined with a burnishing effect to provide the necessary highly-polished mirror surface. Pure aluminum does not oxidize, nor does it require repolishing (which would in itself destroy the accuracy of the surface). It must, however, be kept clean. The typical environment for such a mirror is in a mountain-top observatory, away from most elements capable of causing contamination of the reflective surface. Producing a concave, parabolic surface by machining may be possible with some difficulty and much expense on a 5-axis CNC (computerized, numerically controlled) milling machine. The tool for CNC machining, would necessarily be one with only minimum point contact because of the compound motions necessary to achieve the accuracy of finish required. Such a point would likely cause only burnishing of a concave aluminum surface without actual metal removal. Obviously, the complexity of the 5-axis machine, its programming and its operation would make a simpler and more easily operated machine desirable.