Hard x-ray imaging by Multilayer Zone Plates

Interferometric Position Control

All motorised degrees of freedom (except for the MZP tip/tilt) offer position data by their encoders; in addition, the relative position between sample holder and MZP holder can be measured in real-time and time-stamped to the actual image acquisition using an attocube FPS-3010 interferometer.

The measurement heads are attached to the sample adapter plate, which is connected to the P-733 scanner. The sample itself is mounted on a typical diffractometer pin at the bottom of the plate, the heads are screwed in at a top and side position. The distance between sample and head is below 50 mm.

The laser beams are directed onto the MZP holder as shown in the rendering below; the MZP, situated on a TEM grid, is attached to a diffractometer pin in a distance below 20 mm from the laser spots.

Sample holder with interferometric heads looking at the optics holder (Rendering).

This setup allows to directly measure the relative movement of optics and sample in real-time. The additional mass of the interferometric heads including holding facilities and glass fibre is below 30 g; the sample adapter plate has a mass well below 100 g. The dynamics of the P-733 scanner are rarely affected for scan speeds below 100 Hz.

The interferometric position data can be acquired via a digital interface and processed in real-time by a custom-designed digital electronics device consinsting of an ATmega 1284P microcontroller, shift registers, and SPI-interfaced SRAM memory chip. Using trigger signals, the position data is linked to the actual image acquisition time.

Two modes are envisaged: the 2D position data is averaged over the illumination period (typically 1 ms for the Eiger detector in scanning mode) for each image individually; for longer time periods, a 2D histogram of the movement can be measured and used as a point spread function to (i) estimate the image quality or (ii) correct algorithmically for movements.