CUBES: mechanical design and analysis for high-resolution UV spectroscopy on the VLT

Abstract

We present the mechanical design and analysis of CUBES (Cassegrain U-Band Efficient Spectrograph), a near- UV spectrograph for the ESO Very Large Telescope (VLT). First, we describe the challenges and key concepts of the mechanical design. Then we report on the finite element analysis (FEA) covering modal and seismic analysis, thermal analysis, opto-mechanical flexure analysis, vibration analysis, and bolt verification according to VDI 2230. All analyses are performed with ANSYS 2024/2025 R1 and demonstrate compliance with the applicable ESO requirements. The lowest eigenfrequency of CUBES is 20.1 Hz, exceeding the 15 Hz requirement, while maximum seismic displacements remain below 5mm with positive margins of safety throughout. The opto-mechanical flexure analysis confirms that spot displacements from slit to detector stay well within the 50 μm internal budget, and that the Active Flexure Compensation (AFC) system effectively reduces residual spectral drifts to below ±2 μm after correction. The thermal time constant of the camera lenses is estimated at approximately 11 h, and the vibration analysis of the cryocoolers indicates that the acceptance limits can be met with optimized operational settings.