  We simulate the flux emitted from galaxy halos in order to quantify the
brightness of the circumgalactic medium (CGM). We use dedicated zoom-in
cosmological simulations with the hydrodynamical Adaptive Mesh Refinement code
RAMSES, which are evolved down to z=0 and reach a maximum spatial resolution of
380 $h^{-1}$pc and a gas mass resolution up to 1.8$\times 10^{5} h^{-1}
\rm{M}_{\odot}$ in the densest regions. We compute the expected emission from
the gas in the CGM using CLOUDY emissivity models for different lines (e.g.
Ly$\alpha$, CIV, OVI, CVI, OVIII) considering UV background fluorescence,
gravitational cooling and continuum emission. In the case of Ly$\alpha$ we
additionally consider the scattering of continuum photons. We compare our
predictions to current observations and find them to be in good agreement at
any redshift after adjusting the Ly$\alpha$ escape fraction. We combine our
mock observations with instrument models for FIREBall-2 (UV balloon
spectrograph) and HARMONI (visible and NIR IFU on the ELT) to predict CGM
observations with either instrument and optimise target selections and
observing strategies. Our results show that Ly$\alpha$ emission from the CGM at
a redshift of 0.7 will be observable with FIREBall-2 for bright galaxies
(NUV$\sim$18 mag), while metal lines like OVI and CIV will remain challenging
to detect. HARMONI is found to be well suited to study the CGM at different
redshifts with various tracers.
