6. Simulation and Stacking of GAIA Observations

The simulation of observations and the testing of data analysis procedures on simulated observations is a necessary step in the feasibility studies of a scientific space mission. This is because in most cases the economic effort required by a space mission can only be justified if unprecedented measurement capabilities are to be expected, and as a consequence one usually cannot use real images obtained with different instrumentation for these purposes. This is the case, e.g., for the detection and observation of stars in GAIA Astros, in which case a great effort has been devoted to the generation of synthetic images for use in numerical simulations. Basically, synthetic images are considered useful for testing because a wide variety of conditions can be simulated that could be difficult to obtain from real images, and because images of a given field in several photometric bands can be generated, thus providing simultaneous multi-band photometry, as it will be the case for GAIA.

In the case of galaxy observations, however, the wide range of morphologies, structures and surface brightnesses displayed by galaxies makes synthetic objects, e.g. galaxies from IRAF, not realistic, and the use of suitable real fields are preferable. In this Chapter procedures for the simulation of GAIA galaxy observations and for their superposition into an all-mission ``flux map'' are described. The simulation of observations is based on HST WFPC2 images and on realistic assumptions about GAIA BBP performance with respect to electron count rate, PSF and noise. The superposition, or stacking, of simulated observations into a flux map is then carried out through a procedure that mirrors the one used in the simulation of observations.