  We present the results for the detectability of the O2 and O3 molecular
species in the atmosphere of an Earth-like planet using reflected light at the
visible wavelengths. By quantifying the detectability as a function of
signal-to-noise ration (SNR), we can constrain the best methods to detect these
biosignatures with nest-generation telescopes designed for high-contrast
coronagraph. Using 25 bandpasses between 0.515 and 1 micron, and a
pre-constructed grid of geometric albedo spectra, we examined the spectral
sensitivity needed to detect these species for a range of molecular abundances.
We first replicate a modern-Earth twin atmosphere to study the detectability of
current O2 and O3 levels, and then expand to a wider range of literature-driven
abundances for each molecule. We constrain the optimal 20%, 30%, and 40%
bandpasses based on the effective SNR of the data, and define the requirements
for the possibility of simultaneous molecular detection. We present our
findings of O2 and O3 detectability as functions of SNR, wavelength, and
abundance, and discuss how to use these results for optimizing future
instrument designs. We find that O2 is detectable between 0.64 and 0.83 micron
with moderate-SNR data for abundances near that of modern-Earth and greater,
but undetectable for lower abundances consistent with a Proterozoic Earth. O3
is detectable only at very high SNR data in the case of modern-Earth
abundances, however it is detectable at low-SNR data for higher O3 abundances
that can occur from efficient abiotic O3 production mechanisms.
