Pointing the James Webb Space Telescope through lensing clusters - can the first stars and galaxies be detected?

The James Webb Space Telescope (JWST), scheduled for launch in 2014, is expected to revolutionize our understanding of the high-redshift Universe. Even so, many of the most interesting sources that may be hiding at redshifts z~10 (population III stars, dark stars, population III galaxies) are likely to be intrinsically too faint for JWST. Here, we explore the prospects of searching for the first stars and galaxies by pointing JWST through foreground lensing clusters. Observations of this kind can reach significantly deeper than the currently planned JWST ultra deep field in just a fraction of the exposure time, but at the expense of probing a much smaller volume of the high-redshift Universe. We also present Yggdrasil, a spectral synthesis code for modelling the first galaxies, and use it to derive the masses of the faintest pop I, II and III galaxies that can be detected through broadband imaging in JWST ultra deep fields.