[en] Dipolar ultracold gases are of great interest for quantum simulation experiments. The first step of such a simulation is deep cooling of atoms in a pre-specified internal state. The results of detailed studies of the formation of a spin-polarized cloud of thulium atoms in a dipole trap are presented. We have calibrated the magnetic field by means of radio-frequency Zeeman spectroscopy and measured the polarization of atoms in an optimized trap by the beam expansion method in a magnetic field gradient based on this calibration. We have obtained 1.1 × 10⁶ atoms at a temperature of 27 μK with polarization 〈m_F〉 = –3.95 ± 0.12.