The commonest shape for the primary mirror of a reflecting telescope is a simple paraboloid. It gives excellent images at the centre of the fiat focal plane although the area over which acceptable images are produced can be small. Much of the variety seen in telescope design is a consequence of getting an observer or a detector to the focused image. The most straightforward focal position is the PRIME FOCUS . At this focus, the rays from the object being observed have only been reflected once so that the reflection losses are minimized. Many radio telescopes are of this construction.In the largest optical telescopes there is sometimes provided a small observing cage at prime focus in which the observer arid his equipment remain during an observation. In smaller telescopes such an obstruction of the mirror would be unacceptable and an additional mirror is provided to give the NEWTONIAN FOCUS. This is a traditional configuration often aged by amateur astronomers. Another arrangement is the CA30EGRAJS SYSTEM, where; a small convex secondary mirror intercept* the ray path before prime focus. The rays are then reflected through a hole at the centre of the primary mirror to a focus just behind the mirror. This focus is often more easily accessible than the prime focus and in capable of having much heavier equipment attached .For certain kinds of optical observations, such as high-resolution spectroscopy, special equipment is needed which is much too heavy to he attached directly to the telescope or its mounting. For such observations a COUDE FOCUS is provided some way from the telescope tube. The presence of a prime-focus cage or of secondary mirrors in the primary beam of the telescope does not produce images with holes in their centres. Each part of the beam of radiation from the object contains essentially the same information about the object. If one part of the beam is obstructed by a secondary mirror, for example, the main effect is to reduce the intensity of the filial image. A hole in the image could only be produced by an obstruction in the converging beam after the radiation had been reflected by the primary mirror, and even then only if the obstruction was fairly close to the focal plane of the telescope. The supports of the prime-focus assembly or the secondary mirror are responsible for the cruciform appearance of stellar images. The supports cause some of the starlight to be diffracted out of its original path. These produce the DIFFRACTION SPIKES seen around bright stars on most astronomical photographs.

The first optical telescopes built did not use reflecting mirrors. Instead, a lens was used to focus the light to give an image. The largest existing refractor now in use is the 1-m telescope of the Yerkes Observatory, University of Chicago. Difficulties of mounting and supporting the objective lens so that the image stays fixed and in focus as the telescope moves to follow the sky impose a limit on the size of refracting telescopes which may be built.

One of the biggest disadvantages of most reflectors is that while the images are excellent on the axis of the instrument, they rapidly deteriorate with angular distance from the axis. We say that there is a fairly small FIELD OF VIEW over which the images are of acceptable quality (where the word ‘acceptable’ depends on the type of observations being made). Spherical reflectors suffer from SPHERICAL ABERRATION which causes the rays from the edge of the mirror to come to focus at a different position from the inner rays. Bernard Schmidt of the Hamburg Observatory realized that it was possible to correct for this by placing a thin corrector lens in the path of the rays before they struck the mirror. Such an arrangement produces excellent images over a very wide field, often several degrees across. Telescopes based on this principle are called SCHMIDT TELESCOPES .

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