The Extreme Ultraviolet Imager (EUI) will provide image sequences of the solar atmospheric layers above the photosphere, thereby providing an indispensable link between the solar surface and outer corona that ultimately shapes the characteristics of the interplanetary medium. Scientific topics to be addressed include monitoring the low atmosphere counterparts of large-scale solar eruptive events such as CMEs and the study of fine-scale processes in the solar atmosphere. EUI will also provide the first-ever images of the Sun from an out-of-ecliptic viewpoint (up to 34° of solar latitude during the extended mission phase). The EUI instrument suite is composed of two high resolution imagers (HRI), one at Lyman- and one dual band in the extreme UV alternatively at 174 and 335 Å, and one dual band full-Sun imager (FSI) working alternatively at the 174 and 304 Å EUV passbands, in addition to a common electronics box containing the data processing unit and power supply.
The technology for building the EUI instrument under the challenging constraints of the Solar Orbiter mission is at a high level of maturity and builds on heritage from past missions (Soho/EIT, Trace, Stereo SECCHI/EUVI, Proba-2 SWAP). In the Solar Orbiter EUI units, the image is produced by a mirror-telescope, working in nearly normal incidence. The EUV reflectivity of the optical surfaces is obtained with specific EUV multi-layered coatings, providing the spectral selection of the EUV units (2 HRI and 1 FSI). The spectral selection is complemented with filters rejecting the visible and IR radiation. The UV photons reach detectors (back-thinned active pixel sensor [APS] detectors of 2k x 2k format for the HRI channels and 4k x 4k format for the FSI channel) where they are converted, amplified, and digitized by an A/D converter. For each detector pixel, the resulting signal in DN is proportional to the exposure time and to the solar flux corresponding to the small viewing angle of the pixel in the given band pass.
The HRI and FSI telescopes have spatial resolutions of 1 arcsec and 9 arcsec, respectively. The temporal cadence of HRI depends on the target and can reach sub-second values to observe the fast dynamics of small-scale features. The FSI cadence will typically be of the order of 10 minutes in each passband, but can occasionally reach 10 s. Owing to its high-cadence imaging characteristics, the EUI can produce a much higher data volume than can be down-linked within the available telemetry. Two solutions will be implemented. First, state-of-the-art compression algorithms will be developed; a compression factor up to 50 will be carefully selected for each EUI passband so as to ensure that the compression algorithm does not compromise the targeted features. Second, fully autonomous on-board software will be created to perform an intelligent selection of the most interesting data (e.g., observations of an eruptive event) for transmission to the ground.