Flight-oriented design reconfiguration and diffractive effects analysis of CNT-based entrance filters for the MUSE EUV spectrograph

The Multi-slit Solar Explorer (MUSE) requires large-aperture extreme-ultraviolet optical blocking filters that combine high in-band throughput, strong out-of-band rejection, launch survivability, and limited diffraction. We report the transition from the Structural and Thermal Model (STM) configuration to a qualification-model-oriented entrance-filter design for the MUSE spectrograph. Three 250 mm-class STM filter samples—a baseline Al-coated carbon-nanotube (CNT) pellicle with an embedded CNT mesh, a thicker-Al variant, and a no-CNT-mesh variant—were tested in a representative telescope mock-up. All three remained intact after a 4.54 grms random-vibration exposure, a load representative of qualification levels. Partial membrane failures occurred during a subsequent 6.4 grms higher-margin exposure, with surviving sectors and damage morphology indicating dynamic differential pressure as the dominant load. The CNT-based STM filters therefore satisfied MUSE qualification requirements, though with a tight margin in terms of mechanical resistance to high differential pressure loads during launch. The higher-margin test nevertheless provided a useful stress-to-failure experiment and motivated a conservative design evolution for the qualification models. The revised architecture removes the fine CNT mesh, returns to a thicker CNT pellicle, and subdivides the aperture with a rigid, large-cell metallic hexagonal support having 26 mm clear cells, 1.2 mm meshes, and an 8.6 % geometric blocking factor. Fourier-optics simulations using a realistic MUSE pupil and measured mirror roughness indicate that the support-induced first-order structure remains within the instrument angular sampling and that secondary features are orders of magnitude below the point-spread-function core after detector sampling: thus, removing the issue of diffraction caused by CNT hexagonal-based meshes almost entirely. Updated measured/inferred filter transmittances for the thicker CNT pellicle are 32.4%, 28.6%, and 8.5% at 108, 171, and 284 Å, respectively. The results define a mechanically more conservative qualification-model design with negligible predicted diffraction impact.