CASTOR (The Cosmological Advanced Survey Telescope for Optical and UV Research) is a proposed Canadian-led flagship mission designed to provide high-resolution imaging and spectroscopy in the UV/blue-optical spectral region (0.15-0.55 microns). In the current design, the imager will cover a 0.25 sq. deg. field of view simultaneously in three bands, providing a hundred-fold improvement in survey speed over the legendary, but aging, Hubble Space Telescope (HST). In addition, a multi-object Digital Micro-Mirror (DMD) spectrograph, covering an adjacent field of view, will provide moderate- to high-resolution UV spectroscopy, while a wide-field, low-resolution spectroscopic capability will be provided by a grism. CASTOR's UV imaging and spectroscopic capabilities will enable a vast range of science in the post-HST era, including the physics of star formation from our galaxy to the distant universe, exploring the atmospheres of exoplanets, improving constraints on dark energy, and studying the properties of the outer solar system. CASTOR's data will far surpass any previous UV/blue-optical surveys in terms of sensitivity and angular resolution and will provide complementary capabilities to longer-wavelength data from the Euclid and WFIRST missions, as well as the ground-based LSST. CASTOR will be a strategic asset for Canadian astronomy in the coming decade, showcase the capabilities of Canadian aerospace companies on a global stage, and provide unprecedented opportunities for education and public outreach. A series of CSA-sponsored scientific and technical studies carried out during the past decade have advanced the mission concept to a mature state, and international partners are now waiting for a commitment by Canada.

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