Grism

Simplified working principle of grisms

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. ^[1] 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. ^[2]

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 of the NIRCam instrument) on the James Webb Space Telescope uses grisms. The system allows coarse optical path length matching between the different mirror segments. ^[3]

History

The grating prism was first described in 1973 by Ira Sprague Bowen and Arthur H. Vaughan Jr. in a paper explaining an experiment using a "non-objective grating" located in the convergent beam of a telescope, which allowed to significantly reduce its off-axis aberrations. ^[4] In 1997, this instrument was patented by Chungte W. Chen and Ernest W. Gossett (No 5,652,681), the name grism was chosen because of the combination grating-prism. ^[5]

See also

• Diffraction grating
• Echelle grating
• Slitless spectroscopy

References

1. Palmer, C., & Loewen, E. G. (2005). Diffraction Grating Handbook Newport Corporation 2005, p.265 Retrieved 2025-07-14
2. "Grism developments for space- and ground-based astronomy". spie.org. Retrieved 2025-07-14.
3. Greene, Thomas P.; et al. (2016). "Slitless spectroscopy with the James Webb Space Telescope Near-Infrared Camera (JWST NIRCam)". In MacEwen, Howard A; Fazio, Giovanni G; Lystrup, Makenzie; Batalha, Natalie; Siegler, Nicholas; Tong, Edward C (eds.). Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave. Proceedings of the SPIE. Vol. 9904. pp. 99040E. arXiv: 1606.04161 . Bibcode:2016SPIE.9904E..0EG. doi:10.1117/12.2231347. ISBN   978-1-5106-0187-1. S2CID   119271990.Retrieved 2025-07-14
4. Ira Bowen et Arthur H. Vaughan, Jr. Publications of the Astronomical Society of the Pacific, The University of Chicago Press, 1973, p. 174-176 (DOI 10.1086/129428, JSTOR 40675355) Retrieved 2025-07-14
5. US5652681A,Chen, Chungte W.&Gossett, Ernest W.,"Grism (grating-prism combination)",issued 1997-07-29  Retrieved 2025-07-14

Further reading

• Kitchin, C. R. (July 27, 2020). Astrophysical Techniques (7 ed.). Seventh edition. | Boca Raton : CRC Press, 2020.: CRC Press. doi:10.1201/9780429491139. ISBN   978-0-429-49113-9.
• The Near Infrared Camera and Multi-Object Spectrometer includes a grism http://www.stsci.edu/hst/nicmos/design/grisms
• Deen, Casey P.; Gully-Santiago, Michael; Wang, Weisong; Pozderac, Jasmina; Mar, Douglas J.; Jaffe, Daniel T. (May 8, 2017). "A Grism Design Review and the As-Built Performance of the Silicon Grisms forJWST-NIRCam". Publications of the Astronomical Society of the Pacific. 129 (976) 065004. doi:10.1088/1538-3873/129/976/065004. ISSN   0004-6280. A review of grism design for astronomy.