MULTISPECTRAL SOLAR TELESCOPE ARRAY .4. THE SOFT-X-RAY AND EXTREME ULTRAVIOLET FILTERS

The multilayer mirrors used in the normal-incidence optical systems of the Multi-Spectral Solar Telescope Array (MSSTA) are efficient reflectors for soft x-ray/extreme ultraviolet (EUV) radiation at wavelengths that satisfy the Bragg condition, thus allowing a narrow band of the soft x-ray/EUV spectrum to be isolated. However, these same mirrors are also excellent reflectors in the visible, ultraviolet, and far-ultraviolet (FUV) part of the spectrum, where normal incidence reflectivities can exceed 50%. Furthermore, the sun emits far more radiation in the ultraviolet and visible part of the spectrum than it does in the soft x-ray/EUV. For this reason, thin foil filters are employed to eliminate the unwanted longer wavelength solar emission. With the proper choice of filter materials, the filters can also be used to eliminate EUV radiation at longer wavelengths, where the increasing specular reflectivity of multilayer mirrors and the high intensity of solar emissions can cause "contamination" of the image in the narrow band defined by the Bragg condition. In addition, filters can eliminate higher order multilayer reflections. Finally, filter absorption edges can sometimes be utilized to reduce the width of the primary bandpass. The MSSTA instrument uses various combinations of thin foil filters composed of aluminum, carbon, tellurium, potassium bromide, beryllium, molybdenum, rhodium, and phthalocyanine to achieve the desired radiation rejection characteristics. We discuss issues concerning the design, manufacture, and predicted performance of MSSTA filters.