A study of GOES SXI entrance filter designs and the potential for on-orbit degradation

Abstract

The Solar X-ray Imager (SXI) was launched 24 May 2006 on Geostationary Operational Environmental Satellite (GOES-13). SXI is a grazing incidence X-ray telescope that focuses an image of the Sun onto a CCD detector through a set of selectable filters. The X-ray image data are transmitted at the rate of at least one image per minute, which permits the reconstruction of near-real-time solar images in the 6-60Å range (photon energy 2000-200 eV). Thin film filters consisting of aluminum, titanium, and polyimide are used in the entrance of the telescope to eliminate visible light. During the first six months of on-orbit operations the amount of stray light transmitted increased approximately linearly with time, consistent with the formation of small (less than 50 micron) pinholes. A laboratory investigation was initiated and witness sample filters were subjected to energetic particles simulating the on-orbit radiation environment and their quality was assessed using visible light-leak testing and scanning electron microscope imaging. It was concluded that galvanic corrosion of aluminum and titanium initiates pinholes that subsequently grow in dendritic fashion by spalling off of aluminum to relieve the internal film stress. The test program also revealed that the geostationary radiation dose level can damage polyimide and lead to filter failure. Radiation damage may have been responsible in part for the increased light levels observed in the GOES-12 SXI and with increased exposure a similar observation could manifest on GOES-13 SXI. This paper presents the methodology and results for the entrance filter test program for the GOES SXI telescopes and presents recommended improvements for future instruments.