ELTs, adaptive optics and wavelengths

A number of Extremely Large Telescopes for visual-infrared and adjacent wavelengths are in various degrees of progress. All have primary mirrors with equivalent diameters larger than 20 m and are intended for operation with adaptive optics systems. We discuss several ELT observing parameters as functions of wavelength. Stellar energy distributions and atomic line spectra are inspected as are the transmission of the Earth's atmosphere, the emissivity of the sky and telescope and instruments as well as detector sensitivity, resolution and signal-to-noise ratio. The spatial resolution depending on the size of the diffraction limited adaptive optics point spread function is discussed. We have evaluated the ELT efficiency in terms of Johnson V to N band photometry, simulating diffraction-limited ELT images of a stellar field at 4 Mpc and 4 kpc, respectively. We conclude that the information content at shorter wavelengths is of dominant nature and that there is every reason to do the utmost to include shorter wavelengths in the AO regime. We propose to adopt a short-wave length goal of 1 000 nm for first light AO with later updates reaching down to visual wavelengths.