In spectroscopic observations, sometimes you meet grism, sometimes grating.

Both of them could cause light dispersion, but what is the difference?

  • $\begingroup$ Grism: en.wikipedia.org/wiki/Grism $\endgroup$ Apr 21, 2015 at 10:45
  • $\begingroup$ I know that link. What is the function of a prism? A prism itself is enough to disperse light. $\endgroup$ Apr 21, 2015 at 11:04
  • 1
    $\begingroup$ They disperse light with different dispersion functions, allowing astronomers to see different spectral features. $\endgroup$ Apr 21, 2015 at 11:29

1 Answer 1


Transmission gratings on their own introduce chromatic aberration. This is because they change the effective focal length and do so as a function of wavelength. The chromatic aberration can be eliminated by introducing a prism of the correct dispersion. Known as a grism, the grating/prism combination provides an unaberrated image at the 0th order in addition to the spectrum, although the spectrum has fairly low resolution.


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