General illustration of the spectrograph model and of the purpose of the spectrum extraction, (a) Illustration of the image formation for a monochromatic star observed through a slitless spectrograph, (b) Same for a polychromatic star, (c) Real acquired image: The detector generally is a black-and-white sensor, with noise, a structured and chromatic background, and field star zeroth- and first-order spectrograms. The goal of Spectractor is to extract the colour information from this image to obtain the stellar spectrum.