2005A&A...437.1135C

We present high quality astrometry, along with J, H, and K_s photometry, for 30 southern ICRF quasars observed with the ESO NTT/SOFI at La Silla. Our main purpose is to document the optical counterpart of ICRF quasars, by reporting their position in a stellar reference frame and their near-infrared photometry. Our investigation lies within the scope of extending the (radio) extragalactic reference frame into the optical range. An internal precision generally better than 10mas was obtained in positioning the near-infrared counterpart of these quasars within their stellar surroundings. The actual astrometric accuracy is estimated from the departure between the observed and the radio (VLBI) positions of quasars, and is about 35mas, mainly due to the statistical uncertainties of the stellar reference catalogue (UCAC2). The positional accuracy was improved to 25mas for a subset of 14 quasars observed with extended fields of view, as obtained from mosaics of overlapping images. The 3-band photometry was achieved with a precision most often better than 0.04mag, based on calibration with the 2MASS magnitudes. A spectral index α (F_ν∝ν^α) was derived from the observed quasar fluxes in the near-infrared. With a mean value <α≥-1, this spectral index does not show any significant correlation with the radio spectral index.