Reflecting telescope

A reflecting telescope (reflector) is an optical telescope which uses mirrors to reflect light, rather than lenses to pass light. The British scientist Sir Isaac Newton designed the first reflector circa 1670. He designed the reflector in order to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses.

Focal planes

Prime focus

In a prime focus design, the observer sits inside the telescope, at the focal point of the reflected light. In the past this would be the astronomer himself, but nowadays CCD cameras are used.

Related Topics:
Focal point - CCD

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Radio telescopes often have a prime focus design. The mirror is replaced by a metal surface for reflecting radio waves, and the observer is an antenna.

Related Topics:
Radio telescope - Radio wave - Antenna

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Coudé focus

The Coudé design is similar to the Cassegrain except no hole is drilled in the primary mirror; instead, a third mirror reflects the light to the side, and further optics deliver the light to a fixed focus point that does not move as the telescope is reoriented. This design is often used on large observatory telescopes, as it allows heavy observation equipment, such as spectrographs, to be more easily used.

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