## 4 Discussion  
Multispectral imaging in the visible part of the EM spectrum and longer wavelengths up to thermal infrared is a well-established technique. Wavelength filtering is performed using special filters, combined with suitable detectors, which are sensitive to particular wavelengths. The visible part of the EM spectrum is used for imaging of atmospheric pollutants, water, vegetation, soil, and biological tissues. The IR part is used for imaging of forest fires, geological features, clays, mapping terrain, and night studies. The current advances in nanotechnology require the development of multispectral imaging systems, using wavelengths in the XUV and soft-X-ray region. Therefore, the existing multispectral imaging technology must be extended to such small wavelengths. Towards this, our approach in this work, based on HHG, offers a comb of coherent XUV wavelengths, which can be preferentially filtered in an automated way, without altering their optical spatiotemporal characteristics. We presented an automated system for ultrafast multispectral CDI, demonstrating its capabilities at two different XUV wavelengths, namely 32.2 nm and 47.6 nm. Our results clearly show that the imaging capabilities and the structural details that can be resolved depend on the wavelength in use. Our table-top multispectral CDI system can support a larger number of XUV wavelengths by adding more pairs of multilayer XUV mirrors. These pairs can be placed in the same platform, automatically set to the different pair positions, thus selecting the desired wavelength without altering the XUV beam path towards the object. The use of a semi-infinite gas cell for the high-harmonic XUV generation, aside from the excellent stability of the HHG conditions that it offers, can support laser systems with MHz repetition rates, which are commercially available, providing laser pulse intensities appropriate for HHG. Such high repetition rates are highly desirable in, for example, 3D tomographic imaging due to the large number of images required. While in our work, the shortest wavelength in use was 32.2 nm, future prospects of coherent XUV multispectral CDI will be its extension in the soft-X-ray spectral region utilizing appropriate phase-matching conditions [43,44]. Using such small wavelengths will allow assessing unexplored areas of science, such as imaging of chemicals at their absorption edges, combined with fs time-resolved measurements. Conceptualization, N.A.P., M.B., C.B. and E.P. ; methodology, N.A.P., E.P.B., S.P., A.S., D.Z. and N.K. ; data collection, S.P., A.S., A.G., Y.O., G.A., D.L., D.Z. and N.A.P. ; software, S.P., A.S., Y.O., D.Z., N.K. and A.T.; validation, N.A.P., E.P.B., M.T., V.D., E.P. and C.B. ; resources, N.A.P., M.T., E.P. and C.B. ; data curation, S.P., A.S., M.B., E.P.B. and N.A.P. ; writing, original draft  
Figure 7: (**a**) The object under ultrafast CDI study. (**b**) The ideal CDI result corresponding to the object shown in (**a**). (**c**) CDI result for the object shown in (**a**) using the central wavelength of 47.6 nm. (**d**) Reconstruction of the object shown in (**a**) using the CDI result of (**c**). (**f**) CDI result for the object shown in (**a**) using the central wavelength of 32.2 nm. (**f**) Reconstruction of the object shown in (**a**) using the CDI result of (**e**). The white contour shown in (**d**,**f**) corresponds to the shape of the object shown in (**a**) and was placed to guide the eye. The image size of (**b**–**f**) is 512 \(\times\) 512 pixels. preparation, E.P.B. and N.A.P. ; review and editing, E.P.B., N.A.P., M.B., S.P., M.T., V.D., E.P., N.K. and C.B. ; supervision, N.A.P. ; project administration, M.B., N.A.P., M.T., C.B., E.P., N.K. and V.D. ; funding acquisition, N.A.P., C.B. and E.P. All authors have read and agreed to the published version of the manuscript.