short communications J. Synchrotron Rad. (2014). 21, 1367–1369 doi:10.1107/S160057751401618X 1367 Journal of Synchrotron Radiation ISSN 1600-5775 Received 4 June 2014 Accepted 11 July 2014 # 2014 International Union of Crystallography A high-precision instrument for mapping of rotational errors in rotary stages Weihe Xu, Kenneth Lauer, Yong Chu and Evgeny Nazaretski* Photon Sciences, Brookhaven National Laboratory, Upton, NY 11973, USA. *E-mail: enazaretski@bnl.gov A rotational stage is a key component of every X-ray instrument capable of providing tomographic or diffraction measurements. To perform accurate three- dimensional reconstructions, runout errors due to imperfect rotation (e.g. circle of confusion) must be quantified and corrected. A dedicated instrument capable of full characterization and circle of confusion mapping in rotary stages down to the sub-10 nm level has been developed. A high-stability design, with an array of five capacitive sensors, allows simultaneous measurements of wobble, radial and axial displacements. The developed instrument has been used for characteriza- tion of two mechanical stages which are part of an X-ray microscope. Keywords: high-precision instrument; rotational errors; rotary stages. 1. Introduction X-ray microscopy is a powerful tool suitable for studies of the internal structure and composition of various material systems (Ice et al., 2011; Kaulich et al., 2011; Kang et al., 2013). Recent developments of ultra-brilliant synchrotron facilities stimulated development of X-ray imaging techniques and pushed the imaging resolution down to the nanometer level (Yan et al., 2014). In the soft X-ray regime, two- dimensional full-field imaging with better than 10 nm spatial resolu- tion has been demonstrated using a Fresnel zone plate as the focusing optics (Chao et al. , 2012). In the hard X-ray regime, Huang et al. (2013) reported one-dimensional focusing down to 11 nm using multilayer Laue lenses. By taking a series of two-dimensional X-ray projections at different angles, a three-dimensional image of a sample can be reconstructed revealing its internal structure and composition (Kaulich et al., 2011; Holt et al., 2013; Wu et al., 2012). For ptycho- graphy imaging, 16 nm spatial resolution has been reported in three dimensions (Holler et al., 2012, 2014). For all tomographic measure- ments, particularly via scanning X-ray microscopy, rotational errors play a critical role in defining scientific throughput of a microscope unless actively corrected (Holler et al., 2012, 2014). Therefore, char- acterization of displacements due to wobble, axial and radial runouts in rotary stages prior to the measurements is necessary during development of microscopy systems for three-dimensional nano- tomography. A typical method of characterizing rotary stages includes the attachment of a ball gauge to the center of a stage and monitoring its displacement when rotating by sensors (Noire et al. , 2010). Non- contact displacement sensors (capacitive, optical etc.) are usually employed for high-precision characterization (Marco, 2010; Kim et al., 2013). Unfortunately, such ball gauge systems do not provide simultaneous measurements of displacement errors due to wobble, radial and axial runout errors. In this work, we present a system capable of comprehensive characterization of all rotational errors during the same measurement. By incorporating five capacitive sensors and a diamond-turned reference cylinder, nanometer-scale rotational errors can be measured. A mechanically stiff yet instru- mentally flexible design allows the mapping of rotational errors in stages with different form factors and footprints. 2. Instrument design A computer-aided design drawing and a photograph of the instru- ment are shown in Figs. 1(a) and 1(b). The instrument has a robust yet flexible frame structure to provide stability while allowing char- acterization of rotary stages with different footprints and form factors. Five Lion Precision CPL 190 capacitive displacement sensors with C5S-2.0 probes were arranged in the middle of the beam as shown in Fig. 1(b). Sensors S1 and S2 measure the displacement of the cylinder in the X direction, S3 and S4 in the Y direction and S5 in the Z direction. The rotary stage under investigation is placed on top of a high-precision Kohzu YM16A-S1 X–Y manual linear stage. The linear stage provides 25 mm travel in both X and Y directions and yields 0.5 mm resolution. The manual stage is used to position the rotary stage to be characterized within the working ranges of the capacitive sensors. On top of the rotary stage, there is an assembly of X and Y linear piezo stages (Attocube ECS3030/NUM) with the diamond-turned reference cylinder finally on top of that. Piezo stages are used for the alignment of the cylinder axis and the rotary stage center of rotation. The reference cylinder is 40 mm tall and 25 mm in diameter. It also has a diamond-turned top surface with a machined pin in the center and is used for the initial optical alignment with a CCD camera (BASLER Ace acA1300-30gc). The surface roughness of the reference cylinder was measured with a Zygo optical surface profiler and yielded peak-to-valley values of  11 nm [see Figs. 1(c) and 1(d)]. Since the diameter of the capacitive C5S-2.0 probe is about 2 mm, the localized roughness is averaged and it provides an RMS noise of approximately  2 nm. The system is largely controlled through a LabVIEW interface. The interface communicates with some hardware directly, and indirectly interfaces with the remainder through the Experimental Physics and Industrial Control System (EPICS) channel access protocol.