**5. Experimental setup**

is 3 counts per pixel in average. The camera saturates at 64,000 counts. The model includes a beam stop of 1 mm diameter, a coherence of 80.00% and sample vibrations relative to the

The simulations in **Figure 2** demonstrate how significantly the contrast is determined by the intensity ratio of object and reference, the overall coherence, and the dynamic range of the camera. Within a reasonable range and assuming that the intensity of the incoming flux is not a limiting factor, more absorption of the object and a smaller object hole improve the intensity ratio of reference to object and yield better contrast. That is, the image quality can be

The presence of a beam stop allows for using the dynamic range of the camera to detect signals at high scattering angles, which generally increases the contrast. However, if the beam stop becomes too large, the missing small scattering angle signals (corresponding to constant or low frequency modulations in the hologram) offset the intensity level of the reconstruction. The effect can be seen in **Figure 2** for a beam stop diameter of 2.4 mm. The white domains have now a color comparable to the background. For an even larger beam stop, the effect of ringing can be observed, as illustrated for the 3.2 mm beam stop. The optimum size of the beam stop depends on most of the other imaging parameters and has to be determined from simulations. Finally, vibrations between the sample and the camera have a similar effect as a reduced coherence of the incoming beam [19]: Speckles and interference modulations become blurred and the image contrast reduces. Similar to the beam stop size, the question of at which magnitude

**Figure 2.** Contrast of the reconstruction as a function of imaging parameters. Top row: Object linear transmission, from 0.125 (left) to 1.0 (right) in steps of 0.125. Second row: Beamstop diameter, from 0.4 mm (left) to 3.2 mm (right) in steps of 0.4 mm. Third row: Vibrations (sigma of a Gaussian distribution), from 0 (left) to 28 μm (right) in steps of 4 m. Bottom

improved by coating the specimen with some absorptive layer.

row: Object hole size, from 700 nm (left) to 2100 nm (right) in steps of 200 nm.

camera of 300 nm.

230 Holographic Materials and Optical Systems

The design of a holographic end station should be tailored to the actual scientific question of the experiment. In contrast to many other imaging techniques, holography gives the user a large flexibility for instrumentation in the vicinity of the sample. Because of the absence of optical elements, an area of typically 20 cm radius around the sample is freely available and can be used to apply electromagnetic fields or temperature to the sample. Even optical excitations are possible when using a filter of, e.g., aluminum to shield the camera from this form of stray light. To reduce the effect of vibrations, it is recommended to mount the sample on a rigid holder and fix it onto the vacuum chamber.

**Figure 3** illustrates an example of a chamber optimized for magnetic imaging. The beam first passes through a 1 mm aperture to facilitate a vacuum pressure gradient of two orders of magnitude between the two sides of the aperture. This way the chamber can be operated in pressures of as high as 5 × 10−6 mbar, thus avoiding restrictions to ultra high vacuum components and enabling quick venting and pumping. Carbon deposition on the sample was found to be negligible, even when imaging a single sample for more than one week in such a poor vacuum, at least when using photon energies exceeding 700 eV. The suggested chamber has a quadrupole magnet that can produce fields of larger than 1T at a gap of 2 mm, a shutter to stop the illumination during camera readout, a camera with a beam stop, and a sample held by a rigid mounting.

**Figure 3.** Top-view schematic of a holographic imaging chamber.
