Design of First-Order 121.6 nm Minus Filters.

The equivalent parameters function in Macleod software can be used to calculate the equivalent admittance and phase thickness of multilayers, even when the material is both absorbing and dispersive, and the incidence angle is non-zero. We utilized this software to design first-order 121.6 nm minus filters based on a lanthanum trifluoride (LaF)/magnesium fluoride (MgF) multilayer; a Gaussian-type target curve was also introduced into the design. Minus filters with bandwidths of 10 and 5 nm were obtained and had good visible light suppression and low side-lobe ripples. This designed filter will be fabricated for use in a Lyman-alpha coronagraph and imager installed in the Lyman-Alpha Solar Telescope, which will be launched by China in 2021.