  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 (654
pc) we obtain that the number density of stars is ~0.06 star/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.
