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primary mirrorThe optically worked mirror in a reflecting telescope that faces the observed object and collects its light. The front surface is coated, now usually with aluminum (see aluminizing), to provide a high degree of reflectivity. Technical developments have led to ‘superpolished’ mirrors. Modern mirror-substrate materials are glass/ceramic compounds, such as Zerodur or Cer-Vit, or materials such as borosilicate glass, fused quartz, or Pyrex, all of which have very low coefficients of thermal expansion. The molten glass is first cast into an approximation of the designed concave shape, forming the mirror blank. After cooling, the surface must then be figured, i.e. accurately ground and polished. The designed shape of primary mirrors on most large telescopes is a paraboloid or hyperboloid (see Cassegrain telescope; Ritchey–Chrétian optics); spherical mirrors are used on, for example, Schmidt telescopes. For high-quality images to be produced, the final surface must be polished to an accuracy of within one quarter of the wavelength used. The mirror is supported over its rear face, or from within its structure (see below), and can maintain an accurate profile up to considerable diameters without sagging. See also mounting.
The new generation of ground-based reflectors have very large apertures and consequently high resolution. Technological problems and cost limit the diameter of a single one-piece (monolithic) primary mirror to 7–8 meters. Monolithic primaries are designed to be lightweight to prevent sagging. There are two types. Honeycomb mirrors have an interior honeycomb pattern of glass ribs formed, during casting, between the concave front surface and the flat back plate; this reduces the weight while retaining stiffness and strength. The 6.5-meter mirror of the MMT has such a structure. Meniscus mirrors are thin curved plates, concave on the front surface and usually convex on the back; because they are not very stiff, their shape has to be controlled and maintained by an active optics system. The 8-m mirrors of the VLT are so designed. The primary can alternatively be a segmented mirror, composed of many small mirrors that may fit together (as a mosaic mirror) or not, depending on their shapes. Each segment is individually supported and maneuvered by active optics so that the mirrors act together to form the image. Segmented mirrors can be larger than monolithic mirrors. The Keck Telescopes have 10-m mosaic mirrors. Adaptive optics further increase the resolution of all these designs.
The latest reflecting telescopes may have more than one primary mirror. The mirrors may be mounted together on a single structure, or on independent structures as in the VLT. The light collected may be routed by small mirrors or fiber optics to a central point, where it is combined and recorded. See also interferometer.