To preserve the brightness and coherence of x-rays produced by diffraction-limited-storage-ring (DLSR) and free-electron-
laser (FEL) light sources, beamline optics must have unprecedented quality. For example, in the case of the
most advanced beamlines for the DLSR source under development at the Advanced Light Source (ALS), the ALS-U, we
need highly curved x-ray mirrors with surface slope tolerances better than 50–100 nrad (root-mean-square, rms). At the
ALS X-Ray Optics Lab (XROL), we are working on the development of a new Optical Surface Measuring System
(OSMS) with the required measurement accuracy. The OSMS is capable for the two-dimensional (2D) surface slope
metrology over the spatial range from the sub-mm scale to the clear aperture. Usage of different arrangements of the
OSMS sensors allows measuring the mirrors in the face-up or side-facing orientation, corresponding to the beamline
application. The OSMS translation system and data acquisition software are designed to support multi scan measurement
runs optimized for automatic suppression and compensation of instrumental drifts and major angular and spatial
systematic errors. Here, we discuss the recent results of the OSMS research and development project. We provide details
of the OSMS design and describe results of experimental performance tests of the gantry system. In particular, we show
that the system is capable for measurement repeatability with strongly curved mirrors on the level of 20 nrad (rms). The
high angular resolution of the OSMS rotational tip-tilt stage is adequate for implementation of instrumental calibration
with using the mirror under test as a reference. The achieved measuring accuracy is demonstrated via comparison to
metrology with the carefully calibrated Developmental Long Trace Profiler, also available at the XROL.
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