Nanometer accuracy with continuous scans at the ALBA-NOM

We present the continuous scan operation of the ALBA-NOM as a working mode that allows obtaining low noise in short time, as well as high accuracy measurements. In the traditional step-scan operation, the position of the probe beam is kept fixed while many data points of autocollimator are averaged for noise reduction. This operation mode is very safe, as one has a perfect correspondence between mirror position and measured angle, but it is time inefficient, as it disregards all the data values acquired during motion, and basically averages data values taken under identical conditions. On the other hand, continuous scan is less safe in terms of correspondence between mirror position and slope, especially for NOM systems for which the autocollimator does not accept an electronic trigger. Nevertheless, it is possible to perform independent acquisitions of the autocollimator and of the linear stage data during a scan, and synchronize signals a posteriori. This solves the main problem of continuous scan with a NOM. Continuous scan operation for performing measurements is very efficient for noise reduction per unit time, as it allows integrating every single data value taken by the autocollimator. In addition, it opens the possibility of introducing pitch variations of the mirror between scans. This allows obtaining many independent datasets that can be combined using error suppression techniques to reduce not just noise but systematic errors too. In this paper we report the methods and the main results.