Holographic Concave Gratings (Type 1)

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Type I Holographic Concave Gratings are recorded on spherical substrates, with equidistant and parallel grooves. Their geometric optical properties are the same as classically ruled gratings and are interchangeable with them.

When used in a spectrograph, Type I Holographic Concave Gratings are traditionally disposed on the Rowland circle (i.e., the circle defined by the grating center and the tangential radius of curvature of the grating). The point-source entrance slit is also located on this circle, and the grating forms a spectrum on this same circle, virtually free of defocus and primary coma. Spherical aberration is generally reasonable, yet astigmatism is very significant. As a result of this astigmatism, many Rowland spectrographs offer high resolution but are limited in their light-collection efficiency.