Concept for an extreme ultraviolet sensor with a Fresnel zone plate for the GOES-R solar imaging suite

Abstract

A small, but highly variable fraction of the total solar irradiance lies in the extreme ultraviolet (EUV) spectrum. EUV radiation heats Earth's ionosphere, sometimes disrupting microwave communication and navigation and increasing the drag on satellites in low-Earth orbits. Each of the next series of Geostationary Operational Environmental Satellites (GOES), scheduled to operate from 2012 until at least 2029, will fly an EUV Sensor (EUVS) to measure the solar irradiance operationally in several EUV spectral bands. We propose a novel approach using zone plates (ZPs) instead of the transmission gratings that are now in use. A ZP can be used to form a solar image on a small detector array at a selected EUV wavelength. Since the focal length of a ZP is inversely proportional to wavelength, other wavelengths within the passband of the sensor will be blurred at the focal plane. The ZP can be mounted on a thin-film metallic substrate that can act as a filter, transmitting EUV radiation while blocking light at longer wavelengths. Another thin-film spectral filter on the front surface of the detector can further increase the spectral selectivity of the EUVS and make it less sensitive to defects in either thin film. The circular symmetry of the ZP minimizes the variation in detected signal with field angle and polarization, and its focusing capability allows the detectors to be small, making them easier to fabricate and improving their radiometric performance. ZPs are now used routinely at soft X-ray and short EUV wavelengths.