A time-compensated monochromator for ultrashort high-order
harmonics in the EUV and soft X-ray regions is presented.
The system consists of two grazing-incidence toroidal mirrors
used, respectively, as collimating and refocusing elements
and of two multilayer normal-incidence plane mirrors illuminated
in parallel light that rotate along a vertical axis to
select the operative wavelength, but remain parallel to
guarantee the constant direction of the exit beam. This
is performed by simultaneously rotating and translating
one of the two mirrors along an axis parallel to the exit
direction. The pulse time duration is not altered up to
few femtoseconds.