The EUI instrument is composed of two units, the Optical Bench System (OBS) and the Common Electronics Box (CEB), that are linked by an inter-connecting harness. 

The EUI OBS and EUI CEB are both mounted on the so- called payload ’MY’ panel (a large optical bench) on the -Y side of the spacecraft, together with other remote-sensing instruments. The EUI baffles pierce through cut-outs in the top floor ’PX’ panel of the spacecraft (+X side) where they connect to the corresponding feedthroughs of the spacecraft heat shield.


Layout of Solar Orbiter with one side panel removed such that internal remote sensing instruments are visible

The Optical Bench System harbours 3 telescopes: The Full Sun imager (FSI) and the 2 High Resolution Imagers (HRI).  The two HRI telescopes are two-mirror optical systems while the FSI is a one-mirror telescope, each working in near normal incidence. Spectral selection is obtained with specific multi-layered coatings on the mirrors, complemented by filters that reject the visible and infrared radiation. The two HRI apertures are protected by a common heat shield door. The FSI aperture is protected by a second, separate heat shield door. In addition, each telescope has its own instrument door protect- ing the optical entrance aperture. Operating an EUI telescope thus requires that both the corresponding heat shield door and instrument door are opened.

The EUI Full Sun Imager (FSI), will be imaging alternatively in the 17.4 nm and 30.4 nm EUV passbands, with a 3.8◦ × 3.8◦ FOV, corresponding to (14.3 R⊙)2 at 1 AU and still (4.0 R⊙)2 at perihelion. This FOV is unprecedentedly large for a coronal EUV imager and will have a significant overlap with the Solar Orbiter coronagraph Metis. The large FSI FOV is designed to guarantee that, even for maximal off-points by Solar Orbiter, the full solar disc remains in the field of view. FSI will thus provide a view on the global mor- phology of the solar atmosphere (active regions, coronal holes, quiet Sun) thereby setting the scene in which the first Solar Or- biter science question above will be settled. As required for the second and third science question above, FSI will help us under- stand the initial phase of solar eruptive processes (Green et al. 2018) such as flares, CMEs and dimmings. In particular, the large FOV of FSI will bring new opportunities to study stealth CMEs that originate at high altitudes (Robbrecht et al. 2009) in the corona. For all these reasons, EUI will be an essential build- ing block for the “connection science” to which all Solar Orbiter instruments will contribute.

FSI is accompanied by two high-resolution imagers (HRIs) to image fine structures and small-scale events in the solar atmosphere. The passbands of the HRIs are comparable to each of the passbands of FSI. The HRI imaging in the EUV (“HRIEUV”) at 17.4 nm corresponds to the 17.4 nm channel of FSI. The second HRI, imaging in the Lyman-α line (“HRILya”), shares the same resonance formation process for hydrogen as the 30.4 nm channel of FSI for helium. In this way, EUI covers emis- sion from the high chromosphere to the low corona, both at high resolution and at global scales. The HRIs have been designed to have a 2-pixel resolution of 1 arcsec, which corresponds to a pixel footpri

The (E)UV photons are collected on CMOS Active Pixel Sensors (APS) of 3072 × 3072 pixels of 10 μm each. For the HRI channels the sensors are windowed to 2048 × 2048 illuminated pixels. The FSI and the HRIEUV use the APS sensor in a backside thinned configuration, while the HRILya uses the APS sensor ina front-sided configuration. For each detector pixel, the resulting signal is proportional to the solar flux corresponding to the small viewing angle of the pixel in the given pass band. The electrical signals are converted to digital numbers in the front-end camera electronics, before being compressed and stored in the CEB.