  We introduce the Theoretical Astrophysical Observatory (TAO), an online
virtual laboratory that houses mock observations of galaxy survey data. Such
mocks have become an integral part of the modern analysis pipeline. However,
building them requires an expert knowledge of galaxy modelling and simulation
techniques, significant investment in software development, and access to high
performance computing. These requirements make it difficult for a small
research team or individual to quickly build a mock catalogue suited to their
needs. To address this TAO offers access to multiple cosmological simulations
and semi-analytic galaxy formation models from an intuitive and clean web
interface. Results can be funnelled through science modules and sent to a
dedicated supercomputer for further processing and manipulation. These modules
include the ability to (1) construct custom observer light-cones from the
simulation data cubes; (2) generate the stellar emission from star formation
histories, apply dust extinction, and compute absolute and/or apparent
magnitudes; and (3) produce mock images of the sky. All of TAO's features can
be accessed without any programming requirements. The modular nature of TAO
opens it up for further expansion in the future.
