3.2.3 Point Spread Function and Seeing

Although it can be mathematically defined as a probability density function (see Subection 6.2.2), in practice the Point Spread Function (PSF) of a given observation taken with a given instrument describes the surface brightness distribution generated by a point-like source on the focal plane of the instrument during the observation. Thus, roughly speaking the PSF also describes the angular resolution achieved in an observation, which is degraded by instrumental defects as well as by environmental effects.

The most important contributions to the PSF come from atmospheric turbulence, or seeing, diffraction, geometric and chromatic aberrations. The quality of virtually any ground-based astronomical observations, however, is limited by seeing only. Despite the remarkable progress made in adaptive optics techniques, space observatories presently provide the only means to achieve truly diffraction-limited images, and thus small-scale information on a number of astronomical objects. Most of the outstanding results derived from HST observations are due to its unprecedented angular resolution, obtained with a relatively modest-size telescope.

In galaxy surface photometry, seeing most clearly affect the observations near the galaxy centers. The true surface brightness radial profiles of galaxies, for instance, tend to diverge, as $r \rightarrow 0$, as negative powers of $r$. The effect of seeing is to make the observed profile fainter at small radii and brighter at large radii, introducing a central region of nearly constant surface brightness of size approximately equal to the the FWHM of the PSF. The observation of steep brightness cusps at the centers of nearby galaxies, such as those found from HST observations by [Lauer et al. 1995] and [Byun at al. 1996], have therefore been prevented until recently by this effect. Moreover, since some theoretical models, e.g. King dynamical models, predict internally flat profiles of this kind, it was until recently widely believed that the observed flat profiles indicated true flat profiles rather than artifacts introduced by seeing. Another observational effect due to seeing is the distortion of the galaxy isophotes, namely their systematic rounding with respect to the true ones near the galaxy centers. The overall effect of seeing is thus to prevent the ground-based study of small-scale phenomena in most external galaxies.

In this respect, the high angular resolution and the all-sky coverage provided by GAIA will allow to peer into the cores of a large sample of galaxies, thus yielding an unequaled wealth of information on the inner photometric structure and brightness profile, suitable for statistical studies.