## 2 ARCHES LIBS Module
### Hardware Description
The beam splitter is of the polarizing type, providing around 95% overall reflectivity for the linearly polarized laser beam and between about 45% and 48% transmittance for the collected plasma emission. The achromatic lens is a doublet designed for the visible wavelength region with a focal length of 100 mm. The wobble mechanism is realized by a slanted mount of a flat mirror directly on the shaft of a servo motor, with a slanting angle of 8 degrees between the surface normal of the mirror and the rotation axis. This results in a cost-effective and compact scanning unit that allows sampling along a 3 cm long arc on the target. The reflective coating of the mirror is enhanced silver, providing a good reflectivity of about 96% over the relevant wavelength region of 500 nm to 1064 nm. Furthermore, the mirror is able to withstand the laser beam's peak energy density, which we estimate to be on the order of 1J/cm\({}^{2}\), resulting from the laser beam's second-moment width of about 1.7 mm. The laser spot on the sample has a roughly elliptical shape with a Full Width at Half Maximum (\(FWHM\)) of about 90 um along the major axis and about 40 um along the minor axis as seen in Figure 4. It should be noted that the \(FWHM\) is generally not an ideal measurement unit for multi-mode lasers. However, we chose to use this unit since it is more robust against noise and a reasonable estimate for the size of the laser beam on the target for the purpose of LIBS. The laser-induced plasma's emission is directed back into the instrument using the same mirror and achromatic lens. After passing through the beam splitter, it is coupled into an optical fiber connected to the spectrometer. The fiber coupling lens is an achromatic doublet with a focal length of 19 mm, ensuring full utilization of the numerical aperture of 0.22 of the optical fiber. To protect the spectrometer from intense directly back-scattered laser light, a short-pass filter with a cut-off wavelength of 950 nm is added in front of the fiber coupling lens. This filter has a specified optical density of about 5 at the laser wavelength, providing more than 97% transmission in the 500-940 nm pass band. All optical components are securely mounted in a 16 mm ThorLabs steel cage system, which imparts stiffness and long-term stability for the alignment of the optics. The housing of  
Figure 2: LRU2 with LIBS module taking a measurement during the space analogue mission at Mt. Etna, Sicily. the measurement head is 3D printed from PETG using Fused Deposition Modeling (FDM) technology, providing a lightweight and cost-effective mechanical protection. The imaging ratio between the field of view and the 100 \(\upmu\)m multimode optical fiber is approximately five, determined by the focal length ratio of the focusing lens and the fiber coupling lens. With a field of view diameter of about 500 \(\upmu\)m, the most intense core region of the plasma is captured. The compact Avantes AvaSpec-Mini was selected as the spectrometer due to its low mass, volume, and power consumption--approximately 175 g, 95 \(\times\) 68 \(\times\) 20 mm\({}^{3}\), and 1 W, respectively. This OEM spectrometer employs a symmetrical Czerny-Turner design and an uncooled CMOS linear array with 4096 pixels. It is available in various grating and slit configurations for different wavelength regions from near UV to near IR. The AvaSpec-Mini spectrometer integrated into the ARCHES LIBS instrument is equipped with a 1200 lp/mm grating and a 50 \(\upmu\)m slit. It covers a spectral range of 531-776 nm with a spectral resolution of about 0.6 nm. In this wavelength range, the emission lines of several relevant major and minor rock-forming elements (silicon, calcium, sodium, potassium, lithium, and hydrogen) can be detected. If a different wavelength range is more suitable for the elements of interest, the spectrometer can be easily exchanged with another AvaSpec-Mini, as long as it remains compatible with the used optical parts and their coatings.