Thirty Meter Telescopes and Gravitational Lensing

Diffraction limited 30m class telescopes will play an important role in gravitational lensing studies, coming online in approximately 2015. As imaging telescopes they will complement the ∼6m JWST, probing to smaller angular scales in greatly magnified objects near critical lines and for measuring shear of objects below the JWST angular scale, such as luminous super-star clusters at high redshift. The high source density will allow more detailed mass mapping in the weak lensing regime and will be useful in breaking the cosmology-lens potential degeneracy in strong lensing. As multi-object spectrographs 30m telescopes should provide spectra over the entire optical and near infrared spectrum region. The statistical distribution of redshifts needed to invert projected shear measurements and calibration of photometric redshifts for `tomography' will be available to flux levels around 5-10 nano-Jansky (approx 29.5 m<sub>AB</sub>). However, a one nJy object is expected to require ∼500 hours to acquire a redshift, which is most of the dark time in an observing season. Accordingly `gravitational telescopes' will be an important tool for probing the very faint high redshift universe, magnifying a few square arc-seconds at a time by factors of 10-1000.