As we saw, to observe supernovæ with great shifts towards the red, it is necessary to have major images. In practice, that forces to carry out split observations of the same field. It is thus necessary to summon the various installations to build the images of reference and research. In addition, to prevent that the same defects of camera deteriorate the same objects on the image, one carries out a shift of a few seconds of arc between each installation. It is thus essential to realign all the components on the same grid of pixel.
Pairing of the catalogues