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The grating is the "heart" of a spectrometer because it is the component that breaks up the light into its component wavelengths. The two UVCS gratings (one for the LYA channel, one for the OVI channel) are solid pieces of metal with parallel linear grooves, or "rules" inscribed on them. The rules on each grating are packed very tightly: on the LYA grating there are 2400 rules per mm, and on the OVI grating there are 3600 rules per mm. The LYA grating was etched by holography (i.e., with the use of interference fringes generated at the intersection of two laser beams), and the OVI grating was etched by repeated scrapes of a diamond stylus.
The diffraction gratings disperse incoming UV photons (in the direction perpendicular to the slit length) by slightly different angles, depending on their wavelength. The phenomenon of diffraction "bunches" the light into bright fringes, separated by dark areas; these regions of brightness are called "orders" and are labeled by integer values of m, illustrated below:
The "zero order" (m=0) image does not undergo dispersion, but the others do. UVCS primarily observes photons that have been diffracted in the first and second order (m=1,2) angles. The diffracted angle depends on the product between m and the wavelength of light. Unfortunately, this means that photons from different orders can overlap with one another on the detectors. (For example, the H I Lyman alpha line at 1216 Angstroms is observed in first order at about the same dispersion angle as the Mg X line at 610 Angstroms in second order.) Detailed analysis is required to determine the true wavelengths of spectral features.
The UVCS gratings are not flat, but curved slightly into a "toric" shape. This ensures that a focused image from the entrance slits remains focused when diffracted and projected onto the position of the detectors. The overall geometry of this kind of spectrometer is specified by the "Rowland circle" subtended by the focused slit-image, the grating, and the focused spectrum to be detected:
The UVCS gratings can be rotated to project specific spectral lines on the detectors. The rotation angle of a grating is typically denoted the "grating position."
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