Grism

Simplified working principle of grisms

False color of a young stellar object in a dusty cloud near HH34 constructed from Hubble Space Telescope data – note how the jet near the top of the image is not a smooth spectrum, but a bumpy one with strong emission lines

A grism (also called a grating prism) is a combination of a prism and grating arranged so that light at a chosen central wavelength passes straight through. The advantage of this arrangement is that one and the same camera can be used both for imaging (without the grism) and spectroscopy (with the grism) without having to be moved. Grisms are inserted into a camera beam that is already collimated. They then create a dispersed spectrum centered on the object's location in the camera's field of view.

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.

The dispersed wavefront sensing system (as part the NIRCam instrument) on the James Webb Space Telescope uses grisms. The system allows coarse optical path length matching between the different mirror segments.

See also

• Diffraction grating
• Echelle grating
• Slitless spectroscopy

References

• Kitchin, C. R.: Astrophysical Techniques. CRC Press 2009.
• Space Telescope Science Institute
• Lesson on Spectrograph