<title>In-flight characterization and compensation of the optical properties of the EIT instrument</title>

Abstract

Onboard the SOHO spacecraft, the Extreme UV Imaging Telescope (EIT) is imagin successfully the EUV solar corona since January 96. EIT is a normal incidence telescope, segmented in 4 separate quadrants. Each of those quadrants reflects extreme UV (EUV) light in a narrow bandpass defined by multilayer coatings deposited on the mirrors and by aluminum filters used to reject the visible and IR part of the solar irradiance. The specific configuration of the optical system is generating artifacts that must be compensated in the raw solar images. However, the only information available to improve image quality comes from the continuous survey of the solar corona accomplished in flight by EIT. In-flight image characteristics and instrumental aspects are discussed in this paper, showing how methods can be derived to clean up the EIT data. The current investigations are addressing the internal vignetting, the shadow pattern of grids supporting the focal filters, the determination of the instrumental point spread function and the assessment of the telescope focusing, as well as the relation between those factors.