An astronomical illustration of a spectrograph telescope. Spectrograph layout showing the focal plane of the telescope, the collimator lens, dispersive element i.e. prism or grating, camera lens, and spectral image plane. b Views of a ââ¬Ëstarââ¬â¢ emitting three strong emission lines, first in the focal plane of the telescope where a single polychromatic image can be seen, and secondly in the spectral image plane where three separated monochromatic images are visible. c As for b, but with a slit placed in the focal plane of the telescope to restrict the width of each monochromatic image of the star in the spectral image plane
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