The OVI channel contains an extra mirror between the grating and the detector, so that there can be a "redundant" capability to measure H I Lyman alpha photons in the OVI channel. This mirror, initially suggested by Professor Giancarlo Noci of the University of Florence, takes first-order diffracted light around 1216 Angstroms (which would normally not intercept the OVI detector) and reflects it into the detector.
Note that the direction of wavelength dispersion is reversed by reflection through the Noci mirror. That is, "direct" light has a particular relationship between wavelength and "spectral direction" (i.e., angle of dispersion), but "redundant" light from the Noci mirror has the opposite relationship.
Since 1998, when the LYA channel had to be turned off, we have been very grateful for the Noci mirror because it now gives us the only ability to measure H I Lyman alpha line profiles in the extended corona.
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