  The launch of the James Webb Space Telescope will open up a new window for
observations at the highest redshifts, reaching out to z~15. However, even with
this new facility, the first stars will remain out of reach, as they are born
in small minihalos with luminosities too faint to be detected even by the
longest exposure times. In this paper, we investigate the basic properties of
the Ultimately Large Telescope, a facility that can detect Population III star
formation regions at high redshift. Observations will take place in the
near-infrared and therefore a moon-based facility is proposed. An instrument
needs to reach magnitudes as faint as 39mag$_\mathrm{AB}$, corresponding to a
primary mirror size of about 100m in diameter. Assuming JWST NIRCam filters, we
estimate that Pop III sources will have unique signatures in a colour-colour
space and can be identified unambiguously.
