2 Observations

 

Observations of 21 nearby galaxies, listed in Table 1 of Paper I, were carried out with the Danish 1.54 m. telescope and DFOSC (Danish Faint Object Spectrograph and Camera) at the European Southern Observatory (ESO) at La Silla, Chile, and with the 2.56 m. Nordic Optical Telescope and ALFOSC (a DFOSC twin instrument), situated at La Palma, Canary Islands. The data consist of CCD images in the filters U, B, V, R, I and H$\alpha$. In the filters BVRI and H$\alpha$ we typically made 3 exposures per galaxy of 5 minutes each, and 3 exposures of 20 minutes each in the U band. V-band images of the galaxies are shown in Figs. 4 and 5.

Both the ALFOSC and DFOSC were equipped with thinned, backside-illuminated 2 K² Loral-Lesser CCDs. The pixel scale in the ALFOSC is 0.189$^{\prime\prime}$/pixel and the scale in the DFOSC is 0.40$^{\prime\prime}$/pixel, giving field sizes of $6.5\hbox{$^\prime$}\times 6.5\hbox{$^\prime$}$ and $13.7\hbox{$^\prime$}\times 13.7\hbox{$^\prime$}$, respectively.

The seeing during the observations at La Silla was typically around 1.5 arcseconds as measured on V-band frames. On the images taken with the NOT the seeing was usually about 0.8 arcseconds. All observations used in this paper were conducted under photometric conditions.

During each observing run, photometric standard stars in the Landolt (1992) fields were observed for calibration of the photometry. Care was taken to include standard stars over as wide a range in colours as possible, usually from $B-V \approx -0.15$ to $B-V \approx 1.1$. Some of the Landolt fields were observed several times during the night at different airmass in order to measure the atmospheric extinction coefficients. For the flatfielding we used skyflats exposed to about half the dynamic range of the CCD, and in general each flatfield used in the reductions was constructed as an average of about 5 individual integrations, slightly offset with respect to each other in order to eliminate any stars.