There is a growing interest in the automated characterization of open clusters using data from the Gaia mission. This work evidences the importance of choosing an appropriate sampling radius (the radius of the circular region around the cluster used to extract the data) and the usefulness of additional multiband photometry in order to achieve accurate results. We address this issue using as a case study the cluster Alessi-Teutsch 9. The optimal sampling is determined by counting the number of assigned members at different sampling radii. By using this strategy with data from Gaia EDR3 and with observed photometry in 12 bands spanning the optical range from 3000 to 10000{AA}, approximately, we are able to obtain reliable members and to determine the properties of the cluster. The spatial distribution of stars show a two-component structure with a central core of radius 12-13 arcmin and an outer halo extending out to 35 arcmin. With the derived cluster distance (654pc) we obtain that the number density of stars 0.06star/pc^3^, making Alessi-Teutsch 9 one of the less dense known open clusters. The short relaxation time reveals that it is a dynamically relaxed and gravitationally bound system.