His research interests include black holes and active galactic nuclei (AGN) in the early universe, and he was responsible for discovering a new class of extreme active galaxies at high redshift, with much more powerful X-ray emission compared to their expected optical properties. He has also been responsible for the Hubble imaging in several large extragalactic projects, including the Hubble Ultra Deep Field (UDF), Hubble Frontier Fields (HFF), CANDELS, GOODS, COSMOS and others. He is currently in the NIRCam team, which is one of the imaging cameras on the James Webb Space Telescope (JWST). He has previously worked on Hubble imaging cameras including ACS, NICMOS and WFPC2, and is involved with the Nancy Grace Roman Space Telescope, a new space telescope scheduled for launch in the 2020s which will provide Hubble-quality imaging over areas hundreds of times wider, yielding unprecedented large survey volumes of distant galaxies and AGN across cosmic time.