Spectroscopy

We observed NGC 6951 on the night of 9/10 August 1996 with the ISIS double spectrograph attached to Cassegrain focus of the 4.2m William Herschel Telescope, and the gratings B1200 in the blue arm and R1200, simultaneously, in the red arm. The observations include three wavelength ranges: one range around H5#5 and one around H1#1 (both through the blue arm of ISIS), and a near infrared wavelength range around the Caii triplet absorption lines through the red arm of ISIS. At each slit position angle we took two 1800 s integrations in the red arm, each simultaneous with one 1800 s in the H5#5 and one 1800 s in the H1#1ranges.

The gratings provide a linear dispersion of 0.39 Å/pixel, and the wavelength ranges covered with the TEK1 and TEK2 CCD chips used are 4757-5141 Å, 6493-6881 Å in the blue arm, and 8506-8882 Å in the red arm. We used a slit width of 1 arcsec, that projects onto 2.1 pixels (0.81 Å) at the detector. The spatial sampling along the slit is 0.36 arcsec/pixel. NGC 6951 was observed at three slit position angles: the major axis at 138, the minor axis at 48, and an intermediate position at 84.

The observing was performed under photometric atmospheric conditions. At the WHT the seeing is measured continuously with a guide star; it was slightly variable during the observations of NGC 6951, with a median value of 6#6 arcsec. The phase of the moon was grey.

The reduction of the spectroscopic data followed the standard steps of bias subtraction, flatfield correction, wavelength calibration with a CuNe lamp observed before and after the target, atmospheric extinction correction, and flux calibration using the spectroscopic standards G24-9, LDS749B, BD284211 and BD254644 observed through an 8 arcsec wide slit. Cosmic ray hits were removed individually from the 2D frames. Finally the sky background was subtracted from each frame using the outermost spatial pixels free from galaxy contribution. The two exposures in the CaT range were added together after checking that the spatial and spectral alignment was good to a small fraction of a pixel. FIGARO and IRAF were used throughout the calibration and analysis phases.