X-ray optical objective based on Al and Be compound refractive lenses

X-ray parabolic compound refractive lenses (PCRLs) are widely used to focus with high quality a synchrotron source radiation onto an object in a strongly demagnifying setup. This setup allows to generate a concentrated hard X-ray microbeam with lateral dimensions in the submicrometer range. Beryllium and aluminium PCRLs can be operated in energy range up to 60 keV, they are well adapted to the high heatload of the undulator beams at synchrotron radiation sources and used as the main optical element in various applications for microanalysis and microimaging. Unfortunately, in some experimental setups the single PCRL optical system has drawback consisted in the step-like focusing distances at different energies especially when the number of single lenses is not very large and/or the distance between the PCRL and object plane is fixed. An optical objective with varied focal length consisting of two PCRL's separated by a finite distance allows to create more flexible experimental setup and in some cases provides cost-effective solution by combination of PCRLs made of different materials (for example, all we know that Be lenses are more effective but also more expensive and dangerous than A1 lenses) without significant loss in beam size, depth of field, background, flux, and gain.