Quasi-Stellar Objects (QSOs) are active galactic nuclei so powerful, that their luminosity can surpass that of the host galaxy, making the observation of the latter extremely difficult or even impossible. Usually QSOs appear on images as point-like sources, what, together with the enormous distances, makes them excellent objects for the materialization of reference systems. The Gaia mission from the European Space Agency has as main objective to build a 3D map of the Milky Way, in order to do that it will provide astrometry of unprecedented accuracy. Gaia's astrometric measurements will be referred to a fundamental astrometric frame made of QSOs observed by the mission, however the presence of the host galaxy can increase the uncertainty in the position of the QSO, which is a problem in a mission such as Gaia. To minimize this, the QSO's point-spread function (PSF) is compared with the mean stellar one, through 3 morphological indexes (SHARP, SROUND and GROUND).
We will present an analysis of the morphological indexes obtained for the QSOs of the Sloan Digital Sky Survey-DR7. We are going to explore how these indexes can be used to obtain astrophysical information of QSO+host galaxy, when combined with other information such as redshift, and absolute magnitudes. Our results revealed another source of contamination of the QSO's PSFs. It will also be presented a different approach to obtain absolute magnitudes.