  The preparation of a space-mission that carries out any kind of imaging to
detect high-precision low-amplitude variability of its targets requires a
robust model for the expected performance of its instruments. This model cannot
be derived from simple addition of noise properties due to the complex
interaction between the various noise sources. While it is not feasible to
build and test a prototype of the imaging device on-ground, realistic numerical
simulations in the form of an end-to-end simulator can be used to model the
noise propagation in the observations. These simulations not only allow
studying the performance of the instrument, its noise source response and its
data quality, but also the instrument design verification for different types
of configurations, the observing strategy and the scientific feasibility of an
observing proposal. In this way, a complete description and assessment of the
objectives to expect from the mission can be derived. We present a
high-precision simulation software package, designed to simulate photometric
time-series of CCD images by including realistic models of the CCD and its
electronics, the telescope optics, the stellar field, the jitter movements of
the spacecraft, and all important natural noise sources. This formalism has
been implemented in a software tool, dubbed ASTROID Simulator.
