To align the images, a geometrical image of reference is chosen on which the two periods will be aligned images. One generally chooses the image of worse seeing to avoid degrading this one a little more.
The catalogues of objects are used to build the geometrical transformations making it possible to pass from one image to the other. The first stage consists in pairing the objects of the image of geometrical reference and the current image to be able to calculate the transformation.
Pairing of the catalogues