  Most observations of the solar corona beyond 2 Rs consist of broadband
visible light imagery from coronagraphs. The associated diagnostics mainly
consist of kinematics and derivations of the electron number density. While the
measurement of the properties of emission lines can provide crucial additional
diagnostics of the coronal plasma (temperatures, velocities, abundances, etc.),
these observations are comparatively rare. In visible wavelengths, observations
at these heights are limited to total eclipses. In the VUV range, very few
additional observations have been achieved since the pioneering results of
UVCS. One of the objectives of the Full Sun Imager (FSI) channel of the EUI
telescope on board the Solar Orbiter mission has been to provide very wide
field-of-view EUV diagnostics of the morphology and dynamics of the solar
atmosphere in temperature regimes that are typical of the lower transition
region and of the corona. FSI carries out observations in two narrowbands of
the EUV spectrum centered on 17.4 nm and 30.4 nm that are dominated,
respectively, by lines of Fe IX/X (formed in the corona around 1 MK) and by the
resonance line of He II (formed around 80 kK in the lower transition region).
Unlike previous EUV imagers, FSI includes a moveable occulting disk that can be
inserted in the optical path to reduce the amount of instrumental stray light
to a minimum. FSI detects signals at 17.4 nm up to the edge of its FOV (7~Rs),
which is about twice further than was previously possible. Comparisons with
observations by the LASCO and Metis coronagraphs confirm the presence of
morphological similarities and differences between the broadband visible light
and EUV emissions, as documented on the basis of prior eclipse and space-based
observations. The very-wide-field observations of FSI are paving the way for
future dedicated instruments.
