# Multi-Lens Array Full-Field X-ray Microscopy
## 2 Materials and Methods
### Microscopy Setup  
To show the advantages of full-field microscopy with a multi-lens array, we present a proof-of-principle, which was done at a mid-energy setup using the 9.25 keV Ga K\({}_{\alpha}\)-line. The relatively low photon energy of 9.25 keV has been used, because no setup with higher photon energies was available. Of course, Fresnel zone plates would be better suited for this photon energies [25]. In our case, the 9 \(\times\) 9 multi-focus CRL shown in Figure 2 was used. The individual CRLs are designed for an X-ray energy of 9.25 keV. They have all identical optical layout parameters. The object distance is designed to be 50 mm and the image distance to be 700 mm (measured to the entrance aperture of the CRL). With a  
Figure 2: (**a**) SEM image of a 9 \(\times\) 9 CRL array used for multi full-field X-ray microscopy. (**b**) Parameters of a multi-focus CRL with physical aperture \(A\), radius \(R\) in the apex of the parabola, total length \(L\) of the CRL, length \(L_{\text{n}}\) of a single lens element, thickness \(d\) of the web, air gap \(a\) between two adjacent lens elements, and width \(b_{\text{n}}\) between two entrance apertures. total length of the CRL of \(L\) = 4.7 mm, a magnification of approximately 13.1 is achieved. The aperture of the \(N_{\rm v}\) = 19 vertically focusing and \(N_{\rm h}\) = 20 horizontally focusing lens elements varies, respectively, from \(A_{1}\) = 46 \(\upmu\)m at the first element (i.e., \(N_{\rm v,1}\) and \(N_{\rm h,1}\)) over \(A_{10}\) = 43.1 \(\upmu\)m to \(A_{20}\) = 47.9 \(\upmu\)m. This is referred to as Taille lens and allows for high spatial resolution as well as homogeneous image quality [26]. The average transmission of the CRL is 29%. At 24.2 keV, the average transmission of a suitable CRL (\(N_{\rm v}\) = 136, \(N_{\rm h}\) = 137) with the same focal length would even reach 53%. The theoretical diffraction limited resolution of the setup is \(b_{\rm min}=\lambda/2N_{\rm eff}\) = 0.5 \(\upmu\)m, with the effective numerical aperture \(NA_{\rm eff}\). The complete parameters of the CRL used are summarized in Tables S1 and S2 in the Supplementary Material. The different \(N\) for the two half lenses is used to minimize astigmatism. Therefore, the radius of the horizontally focusing lens elements \(R_{\rm h}\) = 6.09 \(\upmu\)m is slightly larger than the radius of the vertically focusing lens elements \(R_{\rm v}\) = 6.08 \(\upmu\)m. The parameter \(b_{1}\) is 27 \(\upmu\)m, i.e., for the first lens element, and increases continuously for the next lens elements. The air gap \(a\) = 30 \(\upmu\)m between neighboring lens elements is constant. The lenses are designed in such a way that the center-to-center distance of adjacent images on the scintillator is 956 \(\upmu\)m.  
The setup at LRM (Wurzburg) uses a liquid Metallet (Excillum, Kista, Sweden) source. The spectrum of this source and the imaging efficiency in single lens full-field microscopy are discussed in detail in [10]. As an objective lens, the above-described multi-focus CRL was used. A photo of the setup is shown in Figure 3a. In the upper right corner, the position of the metal jet is indicated by a white arrow. In front of the Be-window (not visible) in the copper housing, a 7-axis manipulator for the condenser alignment (1 translation + 6-axis hexapod) is placed. The polycapillary (XOS polycapillary, length 155 mm, entrance angle 0.95\({}^{\circ}\) and exit angle 0.3\({}^{\circ}\), focus size 100 \(\upmu\)m) is used for proper sample illumination. The sample is an absorption grating (period 2.4 \(\upmu\)m, 10 \(\upmu\)m gold thickness, microwords GmbH), which is shown in Figure 3b. For the measurements, a rectangular aperture (50 \(\upmu\)m \(\times\) 50 \(\upmu\)m) was placed as near as possible to the sample, i.e., approximately 1.5 mm. It limits the field of view. Thus, this aperture prevents neighboring images on the detector plane from overlapping.