## 1 Introduction
The calibration range in such applications is specified to be equal to or less than \(\pm\)0.1 mm. There are also solutions in which positioning of the magnets is performed with the use of additional shims with defined thicknesses. In this case, the additional screws responsible for vertical or horizontal displacement of the magnet holder can be used [30]. The application of shims can extend the range of positioning; for instance, at the Shang  
Figure 1: Sketches of design solutions of magnet holders: (**a**) with tuning (1) and locking (2) screws; (**b**) with adjusting screws (1) and nuts (2); (**c**) with wedge (1) and adjustment screw (2) and aluminum holder susceptible to deformation with thinned frame walls (3). hai Synchrotron Radiation Facility, the usage of the shims (magnet dimensions equal to 35 \(\times\) 35 mm (width \(\times\) height)) increased the positioning range of the magnets up to \(\pm\)0.25 mm [30]. The main aim of the presented studies is to design and experimentally verify the magnet holders for the THz undulator in the PolFEL project [31; 32]. In order to achieve maximal flux, at a high \(K\) value, or to reach minimal divergence, for a low \(K\) (depending on the further physical experiment needed), the THz undulator period was assumed to be equal to 16 cm. In such a case, each radiation frequency within the range of 0.5 THz-5 THz is available for \(K\) values greater than 3. The important task was also the maximization of the \(\gamma/K\) ratio for every radiation frequency within that range. In the proposed THz undulator design, the appropriate value of the \(K\) parameter can be achieved by the use of the unusually large magnet blocks within the undulator's Halbach array. The high requirements for the design of the THZ undulator forced the authors to conduct an in-depth analysis of the existing solutions in the area of the supporting structure, as well as magnetic block holders. However, none of the existing solutions found in the literature adequately meet the positioning requirements for the magnet blocks. Finally, the solution proposed and developed for the specific and challenging demands of the THZ undulator is a compromise developed based on existing solutions. The undulator design process was iterative and took into account various aspects and boundary conditions that have not been utilized in previous undulators. This design process of the magnet keeper and its measurements led to the final solution described in the paper. The paper introduces a novel approach involving the use of digital image correlation (DIC) to measure the structural behavior of magnet holders. To date, no application of this method has been found in the literature for measuring the deformation (in the range of 0.001-1 mm) of undulator elements. The DIC method, which was proposed by [PERSON] and [PERSON] in 1982 [33; 34], enables the determination of deformation and strains. This is performed by the analyses of images of the investigated structure covered by black speckles produced on a white background. The accuracy of the DIC methodology applied in the paper was previously successfully confirmed based on the examples of the determination of the development of plastic strains in steel structures [35], evaluation of sub-surface damages on surface strain distribution [36], and elastic deformation in hybrid steel/composite joints under static [37] and dynamic [38] loadings. The paper consists of five sections. The literature review and introduction to the investigated problems are presented in Section 1. A brief description of the THz undulator, prototype of the proposed magnet holder, overview of the numerical models used for the calculation of magnetic forces and structural deformations and description of the experimental studies with the use of dial indicators and the Digital Image Correlation are given in Section 2. Section 3 is divided into three parts. Firstly, the results of the calculation of the magnetic forces are presented (Section 3.1).