ABSTRACT
Practical spectroscopes are usually based upon one or other of two quite separate optical principles – interference and differential refraction. The operating principle of diffraction gratings relies upon the effects of diffraction and interference of light waves. The angular dispersion of a grating is not normally used as a parameter of a spectroscopic system. For those spectroscopes that use gratings at high orders, the grating can still be blazed, but then the light is concentrated into short segments of many different orders of spectra. Higher dispersion can be obtained by using immersed reflection gratings. Pure prism-based spectroscopes will rarely be encountered today, except within instruments constructed some time ago. In the absence of absorption, the emergent intensity at a fringe peak is equal to the incident intensity at that wavelength. Fibre-optic cables are now widely used to connect spectroscopes to telescopes, enabling the spectroscope to be mounted separately from the telescope.
