$\star$ Significant geometrical distortions

The Hubble telescope has sudden modifications during the year 2002, the principal one was the installation of a new camera: ACS . This camera has an effectiveness of detection quite higher than that of the WFPC2 (in band I, used mainly for our measurements, this effectiveness is nearly 5 times better); it has two CCDs of very fine pixelisation (the same one as for Planetary Camera of WFPC2) for sizes of $2000\times4000$ pixels, either a total field of sight of more than 3 minutes of square arcs, or a field of sight 4 times larger than the WFPC2, and 25 times larger than Planetary Camera.

This camera physically did not replace the WFPC2 but the camera JIB 8.4 .

Figure 8.4: Distortions of camera ACS. This figure shows on the right a representation of the sky such as it is observable and on the right the same sky observed with camera ACS.

Figure 8.5: History of the instruments present with in the focal plan of the space telescope Hubble. This figure shows in particular that it is the camera WFPC2 which is with the hearth of the telescope. New camera ACS is shifted compared to this point. Images obtained with this instrument present thus of broad distortions.

This new camera was used to take the images of reference of our supernovæ during the summer 2002 and spring 2003, because the WFPC2 had been still degraded since 2001. The correction of the optical distortions is essential if one wants to align the images of follow-up and these images of reference.