Effect of surface scatter upon the MTF of the solar ultra violet imager (SUVI) telescope

Abstract

The solar UV imager (SUVI) is an extreme ultraviolet instrument that will fly on the Geostationary Operational Environmental Satellite (GOES)-R and-S platforms, as part of NOAA’s space weather monitoring fleet. It will provide important information on solar activity and the effects of the Sun on the earth and the near-earth environment. This instrument will image the full solar disc in 6 EUV wavebands between 303.8 A and 93.9 A. A generalized Cassegrain telescope configuration is employed where six mirror sectors utilize multilayer coatings optimized for the six wavelengths of interest. An aperture shutter is used to select the appropriate sector for observations at a particular wavelength. A thinned, back-illuminated CCD sensor with 21μm (2.5 arcsec) pixels resides in the telescope focal plane. The modulation transfer function (MTF) is usually considered to be the image quality criterion of choice for applications where fine detail in extended images needs to be specified or evaluated. However, the contractual image quality requirement was specified in terms of fractional ensquared energy for a variety of different wavelengths and square sizes. In this paper we will calculate and present MTF plots (as degraded by diffraction, geometrical aberrations, surface scatter effects and detector effects) for each of the SUVI wavelengths of interest. Surface scatter due to residual optical fabrication errors is a major factor limiting the performance at the shorter wavelengths, and the large detector size severely limits the performance at all SUVI wavelengths.