Figures below show the observed spectra for different telescope diameters and 10 hours of observation time. The Venus analog was placed 10 pc away. The spectral range observed is the same as the one proposed for the spectrograph in the LUVOIR concept mission. The instrument parameters are based on the work of Robinson et al. (2016).
Figure on the left – 8 meters telescope diameter (D): The blue points represent an inner working angle of 3lambda/D, which would only allow for observations at wavelengths shorter than 1 microns. It would be difficult to characterize the atmosphere from the data corresponding to the blue points, where the only information observed would be the reflected light from the cloud structure. With an inner working angle of 2lambda/D (green points) we would be able to capture the H2O feature at roughly 1.2 microns. The red points represent an inner working angle of lambda/D.
Figure on the right – 15 meters telescope diameter (D): significantly improves the performance of the instrument, it would make it possible to observe the H2O and CO2 features with all the inner working angles tested: 3lambda/D (blue), 2lambda/D (green), and lambda/D (red).