Optical imaging

Optical images through two continuum filters, B and I, were taken with the Brocam II camera on the Cassegrain focus of the 2.56m Nordic Optical Telescope during the nights of 8 and 9 October 1996. B is a special purpose made filter with a square spectral response centered at 4630 Å and with a FWHM=290 Å, so that it does not include any bright emission line. The equivalent width of any emission line within the I filter is negligible. Imaging through emission line free filters is important to ascertain the true circumnuclear stellar structure, that can otherwise be significantly distorted with standard broad band filters, where the emission lines can make an important flux contribution (Vila-Vilaró et al. 1995). The detector was a 10247#71024 Tektronix chip with a sampling of 0.176 arcsec/pixel and a 37#73 arcmin field of view. Weather conditions were photometric and the average FWHM of the seeing profile is 0.8 arcsec. The journal of observations is included in Table .

The reduction and calibration of the images has been carried out in IRAF. Bias and flat field corrections were done in a standard way. The task COSMICRAYS was used for the overall cosmic ray removal.

The final image of NGC 6951 for a given filter is obtained by adding all the images for that filter properly aligned, sky subtracted and calibrated. The alignment, using cross-correlation techniques involving only bright stars, is made to an accuracy of 0.1 pixel. The sky background is estimated by averaging the median flux in a few 407#740 pixel patches located outside the galactic disk. The 38#8 background limit magnitudes are 20.7 and 21.0 mag arcsec^-2 in B and I, respectively.