More information about the HRM can be found here.

Context

In additional to the fundamental wavelength, the generation of x-ray by electron passing through undulators also produces higher order radiation (only the odd orders in fact). For example, for a fundamental at 5keV, photons at 15keV, 25keV, etc. are also generated.

The intensity off successive orders drops very quickly, but the 3rd harmonic is still intense enough to create spurious and unwanted signals. Because photons of higher energy penetrates more in materials, the rejection of these higher order is not straight-forward. This in fact make the problem worse if the attenuator is used, as the fundamental is more attenuated than the higher orders, hereby worsening the ratio of "good" photons to unwanted ones. Moreover, because they are multiple of the fundamental energy and thus share similar diffraction conditions, diffraction-based approach do not work.

The solution is thus to use a pair of Silicon mirrors, which will reflects the fundamental while transmitting most of the third (and above) harmonics. This results in an offset position of the fundamental beam, as shown in the drawing below.