Re: Star Analyzer Prism Myths

Ken Harrison

The grism (gratings-prism) was proposed by Edwin Carpenter back in 1963.
The Carpenter prism has the front face square to the incoming light beam  (no deviation or dispersion), at the rear prism face a transmission grating is mounted (cemented) and as the light exits the prism there is deviation but no dispersion. The angle of the prism and it's deviation angle is designed to bring the spectral wavelength of interest back onto the optical axis.
This makes the mechanical and optical design of the spectrograph much easier. All professional transmission grating spectrographs make use of the Carpenter Prism. (I assume the ALPY also uses the Carpenter prism.)
Edwin F Carpenter at Steward Observatory describes the "grism" constructed and used on the 36" telescope in 1963.
See Daniel Schroeder "Astronomical Optics", Chapter 15, Plane grating Spectrometers, Section iii NonObjective grism, He describes how the inherent coma and defocus at the ends of the range (field curvature) can be significantly improved.
The following patent describes the Carpenter prism at some length but not the source of the name....
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....

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