Through the Looking GLASS (GLASS-JWST), is a James Webb Space Telescope (JWST) Early Release Science (ERS) program led by Prof. Tommaso Treu at the University of California, Los Angeles (UCLA). The two main science theses of GLASS-JWST are to (i) understand the Reionization process of the universe less than 1 billion years after the Big Bang, and (ii) understand how gas and heavy elements are distributed within and around galaxies over cosmic time.

GLASS-JWST achieves this by combining the natural magnifying power of gravitationally lensing by the massive Frontier Field galaxy cluster, Abell 2744, with JWST's unprecedented instruments (NIRISS, NIRCam and NIRSpec) to measure detailed properties of distant galaxies in the early universe.

GLASS-JWST observes the Abell 2744 cluster in two modes. The first (completed as of June 29, 2022) is over the central portion of the cluster with NIRISS Wide-Field Slitless Spectroscopy (WFSS) in three different filters (F115W, F150W, F200W) in order to obtain continuous, R~150 spectra over the entire ~1.1-2.2 micron wavelength range. Simultaneously, JWST obtained imaging over parallel fields in seven different wide-band filters (F090W, F115W, F150W, F200W, F277W, F356W, F444W). The second mode will consist of NIRSpec Multi-Object Spectroscopy (MOS) for a sample of sources identified in the primary NIRISS field, with three different setups (G140H/F100LP, G235H/F170LP, and G395H/F290LP) affording complementary high-resolution (R~2700) spectra. As with the NIRISS, the NIRSpec observations will also be conducted with NIRCam parallel imaging using the same filters.