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M74050
1/4M IMAGING SPECTROGRAPHS
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8 GETTING LIGHT INTO A MONOCHROMATOR
8.1 ACCEPTANCE PYRAMID
The figure below shows the optical path of the light input to the instrument. The spectrograph has
an acceptance pyramid, often described by an F/#. The position and dimensions of the internal
optics determines the pyramid. The optical equivalent is a grating image located behind the slit as
shown in below. Only light that passes through the slit in the direction of this grating image is useful.
Always fill the acceptance pyramid of the instrument with light.
Figure 19: Acceptance Pyramid of Spectrograph
Figure 20: Grating Correctly
“Filled” with Light