The GEO-X optical blocking filter

Abstract

We have been developing an optical blocking filter for GEO-X which will perform soft X-ray imaging spectroscopy of Earth's magnetosphere. GEO-X will provide an unprecedented view through X-ray observations in terms of the interaction between solar wind and magnetosphere. GEO-X consists of MEMS X-ray mirrors and a focal plane CMOS detector which is sensitive to visible photons necessitating a thin-film optical blocking filter to attenuate noise from out-of-band photons while providing high transmittance for in-band soft X-ray photons. The OBF design for GEO-X spans an aluminum 40 mm x 40 mm aperture with 200 nm polyimide and 150 nm aluminum. A square-celled stainless steel mesh with a grid size of 3 mm and a width of 200 um provides mechanical support against launch stresses. The material combination provides high transmittance across the GEO-X bandpass and tolerances for severe launch and space environments at the same time. We conducted performance tests for the EM coupons. We confirmed the pinhole frequency of the aluminum film is very small (visible transmittance < 1e-8). It is found that film uniformity is very good, varying by 1% in polyimide and 2% in aluminum thicknesses over the 20 mm x 20 mm scan region at 277, 380, 522, and 776 eV. Before and after the UV irradiation test (50 ESD), no significant differences in terms of the pinhole frequency are found. Additionally, there are no significant descrepancies in the size of the pinholes for all of the pinholes we investigated before and after the high-speed atomic oxygen irradiation test with a fluence value of 10 19 atoms cm −2 even though additional pinholes were detected after the test possibly due to accelerated metal outgassing contaminants remaining the vacuum chamber.