Optical blocking filters for the Multi-slit Solar Explorer mission: in-band extreme ultraviolet and soft X-ray transmittance spectroscopy measurements and analysis

Extreme ultraviolet (EUV) filters are critical components in solar space missions, ensuring the required wavelength selection and visible light attenuation. As part of the advancement for the NASA Multi-slit Solar Explorer (MUSE) mission, we present a detailed in-band EUV and out-of-band soft X-ray transmittance characterization of small samples of filter candidates representing two design choices for the mission: a baseline design based on heritage filters from past successful missions and an alternative design employing carbon nanotube (CNT) pellicles as substrates. Our aim is both to present the experimental EUV/soft X-ray transmittance measurements and, through proper data analysis, to retrieve the atomic areal densities and the inner layer thicknesses of each sample, including the quantification of the metal oxide layers for each different substrate. The measurements demonstrate the in-band transmittance performance of both heritage and CNT-based filters across EUV bands. These results provide useful input for the construction of the response functions for MUSE and similar EUV space missions. Furthermore, the methodology establishes a framework for the optimization and characterization of thin-film materials tailored as EUV entrance filters for space-based solar observations.