In case a large serie of image is available, it is useful to stack the best seeing images in order to build a high quality reference image for image subtraction. If you do not have many images, just take the best image. In case you have for instance 100 images, it can be very useful to stack the 10 best seeing images. Be careful that none the image you wish to co-add have large defects. Once you selected a list of image, you can run the stack program which will perform the co-addtion. To run the program: