Spectractor extracts spectra from slitless spectrophotometric images and measures the atmospheric transmission on the line of sight if standard stars are targeted. It has been optimized on CTIO images but can be configured to analyze any kind of slitless data that contains the order 0 and the order 1 of a spectrum. In particular, it can be used to estimate the atmospheric transmission of the Vera Rubin Observatory site using the dedicated Auxiliary Telescope.