$\star$ Instrumental Seeing

As we saw in the chapter of detection, there are two contributions to the spreading out of the signal of stars: atmosphere and the instrument. In space, it is primarily diffraction by the optics of the instrument which determines the seeing and the PSF. The profile of stars is thus constant from one observation to another. This makes it possible in practice to build models of PSF which could be used during photometry. The second advantage is that it is not necessary any more to homogenize PSFs between the various observations. The analysis is thus simplified.

However, the presence of the geometric aberrations which involved the spectacular repair of the instrument returned the profile of the relatively complicated PSF. These aberrations were corrected by modifying the optics of camera WFPC2 so as to produce geometric aberrations of its optics in order to compensate for the aberrations of the principal mirror.

Figure .

Figure 8.2: This figure shows the space profile of a star for the space telescope Hubble and Planetary Camera of instrument WFPC2. This PSF was simulated using the TinyTim software in filter F814W in the medium of the CCD.