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Redshift has been used to map the expanse of the universe but what mechanism do scientists use to obtain and single out the light from a galaxy millions of light years away on a prism for getting the spectral lines? Isn't the light extremely feeble?

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A device that concentrates light to make dim distant objects visible is a telescope. Reflector telescopes, which use mirrors, work better for distant objects than refractor telescopes, which use lenses.

A large parabolic mirror collects light and concentrates it towards a nearby point. A small mirror reflects the concentrated light in a useful direction, such as to an eyepiece. There may be more steps along the way, depending on the size and delicacy of the instrument, but the key is taking as much light as possible coming from one small solid angle of sky by using the largest possible collector, and then concentrating it to a small area to produce a clear, bright image.

Extremely distant objects may be redshifted out of the visible light spectrum. For these, astronomers use infrared telescopes. Infrared telescopes work the same basic way as other reflector telescopes, although they look different and use different technologies to do the same thing.

Once the light has been concentrated by a telescope, if a spectrum is wanted, the concentrated light is passed through a slit to another parabolic mirror, which reflects upon a diffraction grating. The diffraction grating separates the light into its spectrum. Another parabolic mirror directs the separated light onto a calibrated photodetector that is more sensitive and accurate than the eye. A computer interprets the photodetector results and outputs the spectrum as a graph. The spectral lines are spikes on the graph.

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