$\star$ The chart of the cosmic ones

Contrary to the stars which have slow and continuous space variations, the cosmic ones present on the images of the fast and discontinuous variations. The cosmic rays touch 1 to 2 pixels in general. A filter Laplacien $\cal L$ (equation 7.20 ) thus makes it possible to select the pixels touched by the cosmic ones. For more precision on this method, one will refer to dokkum2001.

$$\cal L = \begin{pmatrix}0 & -1 & 0\\ -1 & 4 & -1\\ 0 & -1 & 0\\ \end{pmatrix}$$ (7.20)

The pixels which exceed a certain threshold (typically $\cal L$ are compared with the median of the nine adjacent pixels. One second cut dependent on the seeing and the bottom of sky is applied. This second cut makes it possible not to eliminate the pixels from stars and the galaxies. The selected pixels are declared cosmic and are labelled in the chart (binary) the cosmic ones. This procedure is carried out in an iterative way so as to label the cosmic ones spread out over several pixels.