3 Observations

Narrow band imaging in the H$\alpha$ emission line ( $\lambda = 6562.8$ Å) of galaxies was obtained in 1998 and 2000 at the 1.20 m Newton telescope of the Observatoire de Haute Provence (OHP; France) and in 1999 at the 1.23 m telescope of Calar Alto (Spain). The f/6 OHP telescope is equipped with a thinned TK $1024\times1024$ pixels CCD detector. The pixel size is 0.69 arcsec. At the adopted gain, the electron/adu conversion is 3.5 e^-/adu, with a readout noise of 8.5 e^-. The f/8 Calar Alto telescope is equipped with a SITe 2048$\times$2048 pixel CCD detector. The pixel size is 0.50 arcsec. At the adopted gain, the electron/adu conversion is 3.5 e^-/adu, with a readout noise of 5.2 e^-. A total of 26 nights at the OHP and 8 at Calar Alto were allocated to this project. Of these, 22 were totally or partly useful due to technical problems or weather limitations, as reported in Table 3 (logbook of the observations and CCD technical data).

 

 

Table 3: Logbook of the observations.

Telescope	Date	Nights (ass./used)	CCD	Pixel size
1.2 m OHP	25/2-9/3/1998	12/6	TK $1024\times1024$	0.690
1.23 m CA	14-21/4/1999	8/4	SITe $2048 \times 2048$	0.502
1.2 m OHP	7-12/2/2000	7/5	TK $1024\times1024$	0.690
1.2 m OHP	6-12/3/2000	7/7	TK $1024\times1024$	0.690

Each galaxy was observed through two narrow band interferometric filters (see Table 4), one of them including the redshifted H$\alpha$ line (ON) and the second one mesuring the red continuum near H$\alpha$ (OFF). The filters given in Table 4 are used as ON or OFF band for each target, as specified in Cols. 2 and 3 of Table 5. The flux from the [NII] emission lines at $\lambda$6548 Å  and $\lambda$ 6584 Å  is included in the ON band observations. The tipical integration time was of 30-45 min ON- and OFF-band, generally split in 3 shorter exposures. Observations were obtained during poor seeing conditions (2-4 arcsec), expecially at the OHP. The images of a few nearby galaxies have been obtained by mosaicing several frames: M 81 (6 positions), M 106 (2 positions) and NGC 2403 (5 positions).

The observations were calibrated using the standard stars Feige 34 and Hz44 from the catalogue of Massey et al. (1988). Observations of the standard stars were repeated every 2 hours, with an integration of 2 min with the telescope defocused to avoid saturation. Repeated measurements gave <0.05 mag differences, which we assume as the typical uncertainty of the photometry given in this work. Not all frames were obtained in photometric conditions. When the zero point was varying by more than 0.1 mag due to cirrus, we choose to observe only galaxies with could be calibrated a posteriori on available photometry. The determination of the H$\alpha$ equivalent width can however be achieved also in non photometric conditions using field stars to normalize the ON and OFF band images, as described in the next section and in Paper I.