Grism

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A grism is a combination of a prism and grating arranged to keep light at a chosen central wavelength undeviated as it passes through.

The resolution of a grism is proportional to the tangent of the wedge angle of the prism in much the same way as the resolutions of gratings are proportional to the angle between the input and the normal to the grating.

Grisms are normally inserted into a collimated camera beam. The grism then creates a dispersed spectrum centered on the location of the object in the camera field of view.

[edit] Source

Space Telescope Science Institute