$\star$ The coaddition

The image `` summoned '' and its weight are written simply:

$$I_{i,j} = \frac{\sum w^l_{i,j} \gamma_l I^l_{i,j}} { \sum w^l_{i,j} \gamma_l^2}, \ \ \ w_{i,j} = \sum {w^l_{i,j} \gamma_l^2}$$

An example of coaddition is presented in figure 7.17 . It is noticed that all the artifacts present (satellite, cosmic and dead columns) disappeared from the major image. The chart of weight of the major image is presented on the left. The darkest zones are the zones which were eliminated at the time of the coaddition, we recognize the figures of the various artifacts there.

Figure 7.17: This figure has the result of a coaddition. On the left, the major image where the artifacts disappeared. On the line the chart of the weights where the darkest pixels are the pixels of weaker weights. In particular, the figures of the various artifacts are recognized.

This method does not require to have a great number of images to be applied. In particular, it is possible to build a sum of two images where the artifacts are eliminated. Moreover, it is possible to build the chart of weight of the coaddition which could be used thereafter at the time of the subtraction and for calculation of fluxes of the objects.