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The Extreme UV Imaging Telescope (EIT) instrument is operating on-board the SOHO spacecraft since January 1996. EIT is providing EUV observations of the solar corona in four narrow channels: 171, 195, 284 and 304 Å. Due to continuous exposure to the EUV solar irradiation, the instrument performance is continuously evolving. The backside thinned detector is showing important changes in its overall response and local damages of EUV highly exposed areas. These performance modifications can be characterized through several observation analyses that are discussed in this paper. Two major effects are identified: contamination on the detector surface and charge mobility changes in the CCD produced by the EUV irradiation. To restore the instrument response, bakeouts are regularly planned as well as specific observation sequences that are used to characterize the detector damages. An overview of the instrument response behavior is presented in this paper. To cross calibrate this instrument, the spare components of the EIT program were assembled in a new structure used as the EIT Calroc Sounding Rocket instrument. The Calroc flight successfully occurred in October 97 and gathered very useful data that provide additional means to cross calibrate the EIT-SOHO instrument. The Calroc results and their use to improve the EIT-SOHO data are discussed. This sounding rocket mission demonstrated good correlations of the performance monitoring analyses on the EIT-SOHO with the diagnostic deduced from the Calroc observations.

<title>In-orbit performances of the EIT instrument on board SOHO and intercalibration with the EIT Calroc Sounding Rocket program</title>