The ratio of horizontal to vertical source size in storage rings typically is of the order 10. This produces a strongly elongated spot on the sample which is inconvenient for many experiments. This problem can be solved by using 1D lenses from RXOPTICS. These have refractive power in one direction (vertical or horizontal) only. Another example on preconditioning with 1D lenses can be found in the section on coherent illumination.

Fig. 1. Astigmatic focusing in cross-geometry

In the following, we report on an experiment conducted by A. Snigirev, I. Snigireva, I. Dolbnya, and K. Sawhney in collaboration with the optics group at Diamond Light Source. A circular focal spot was generated by astigmatic imaging (that is, with different focal lengths in the horizontal and the vertical direction) of the source via two independent 1D CRLs in cross-geometry (Figure 1). The vertically focusing CRL, placed at 44 m from the source, consisted of 17 beryllium lenses with R = 0.3 mm with an image distance of 4079 mm. The horizontally focusing CRL, placed at 48 m from the source, was made up of 17 beryllium lenses with R = 0.2 μm and 15 beryllium lenses with R = 0.3 μm with an image distance of 1628 mm. The detector was placed 1.4 m behind this second CRL.

Fig. 2. Intensity profiles (by Snigirev et al.)

The gain with this configuration, that is, the ratio of the intensities in focal spots of the same size produced by the CRL on the one hand and by a pinhole on the other hand, turned out to be 1200. Figure 2 shows the vertical and horizontal intensity profiles measured by the detector, including a Gaussian fit (solid line). In both cases the measured width was 7.5 μm (FWHM); hence, the generated focal spot was indeed circular.