The light coming from the entrance slit is transformed into a collimated bundle by a first mirror, which is diffracted from the grating and focused on the exit slit by a second mirror.
At the exit slit, the light then has a certain wavelength which can be changed by rotating the grating. The arrangement thus functions as a monochromator. We speak of a polychromator as used in grating spectrographs when the rotation is eliminated and a photo plate or photo diode array is used in place of the exit slit.
Then the various colors of the spectrum are displayed next to each other simultaneously on the photo plate. Selecting particular curve radii for the mirror as well as particular gratings and mirror orientations permits compensation of the coma and good images with high light conductance.
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