  We research the requirements of High Contrast Imaging when combined with the
cross correlation (CC) of high resolution spectra with known spectroscopic
templates for detecting and characterising exoplanets in reflected light. We
simulate applying the technique to a potentially habitable Proxima b-like
planet and show that the O$_2$ A-band spectral feature could feasibly be
detected on nearby rocky exoplanets using future instruments on the ELT . The
technique is then more widely analysed showing that detections of planets and
O$_2$ with signal to noise in the CC function (SNR$_{CC}$) > 3 can be obtained
when the signal to noise of the simulated planet spectrum (SNR$_{spec}$) is
from 0.25 to 1.2. We place constraints on the spectral resolution, instrument
contrast, point spread function (PSF), exposure times and systematic error in
stellar light subtraction for making such detections. We find that accurate
stellar light subtraction (with 99.99% removal) and PSFs with high spatial
resolutions are key to making detections. Lastly a further investigation
suggests the ELT could potentially discover and characterise planets of all
sizes around different spectral type stars, as well as detecting O$_2$ on
Super-Earths with habitable zone orbits around nearby M stars.
