To obtain the spectra of stars, a prism or a diffraction grating is used as dispersing elements. Why can the prism be installed both in front of the telescope objective and behind it, in the spectrograph, and the diffraction grating – only in the spectrograph?
With the help of a prism located in front of the lens, images of the spectra of all stars in the field of view appear in the focal plane of the lens. A diffraction grating installed in front of the lens will provide images of stellar spectra of several orders of magnitude, which will greatly complicate the picture obtained. Moreover, there are no large diffraction gratings.
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