We observed a sunspot almost exactly at disc centre on 23 September, 2003, with the Tenerife Infrared Polarimeter (TIP; Martínez Pillet et al. 1999) at the $\phi=70$, cm Vacuum Tower Telescope (VTT) on Tenerife. The highly symmetric spot passed the central meridian just at the time of observations; its solar latitude of $9^\circ$ N was almost perfectly compensated by the inclination of the solar axis $B_0=7^\circ$. We took the full Stokes vector through the line profiles of Fe I 10783 Å, Si I 10784.5 Å and Si I 10786.8 Å. The spatial sampling was adapted to the pixel size of 0.4 arcsec width; the VTT correlation tracker largely compensated image motion. The Stokes spectra were corrected for the dark and the gain matrices, as well as for instrumental polarisation and Stokes cross-talk. The spectral resolution finally achieved with TIP and its detector amounts to 32 mÅ.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html