A mirror is an optical device that uses reflection to deflect and focus the path of light rays, producing either a real or virtual image of a source object.

Left: A flat (or plane) mirror creates a virtual image of any object placed in front of it.
Right: The virtual images are the same distance from the mirror as the original objects.

In astronomy, mirrors are used in telescopes to both focus the light from distant stars – often with parabolic mirrors, but also to deflect light to instruments such as spectrographs. Because of losses associated with every “bounce” of light, instrumentation scientists endeavour to minimise the number of mirrors between the primary mirror and the final detection instrument.