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3 Observations

The CCD images of the HCG sample were obtained during three different observing runs (November 1995, April 1996 and February 1997).

Observations have been carried out at the 2.1 meter telescope (design Ritchey-Chretien) at the National Observatory of Mexico in S. Pedro Martir (SPM). The SPM Cassegrain focus (f/7.5) was coupled with a Tektronix CCD of $1024\times 1024$ pixels, each $24~\mu{\rm m}\times 24~\mu{\rm m}$. The telescope scale (13 arcsec/mm) and the pixel dimensions provide a pixel size of 0.3 arcsec/pix with a resulting field of view of $5.12'\times 5.12'$. The CCD gain is 4 e-/ADU.

During these three runs we observed 31 HCGs. All images were obtained with seeing conditions in the range 2-2.6 arcsec. For each HCG two CCD images were taken: the on image, by using a narrow-band interference filter (H$_\alpha^{\rm on}$ filter) centered on the wavelength of the H$_\alpha$ line redshifted to the z of the galaxy (which isolates the H$_\alpha$ emission-line and underlying continuum), and the off image, by using another interference filter (H$_\alpha^{\rm off}$ filter) of similar bandwidth but centered on an adjacent region of the spectrum (isolating continuum light only). Table 3 describes the features of narrow band filters used in this work. In the third column the range of recession velocity that a galaxy should have to give out its H$_\alpha$ line through the interferential filter is shown.

In order to calibrate our data, we have observed some spectrophotometric stars, equally spaced in time during each night, from the list of Massey $\&$ Strobel (1988). Table 4 lists the standards used. The spectrophotometric standards were observed in the same H$_\alpha$ narrow-band interference filters used to observe Hickson Compact Groups.

The flux from [$N_{\rm II}$] emission lines ($\lambda=6548~{\mbox \AA}$ and $\lambda=6584~{\mbox \AA}$) is included in the on observations. Therefore the flux and luminosity here estimated refer to the sum of H$_\alpha$ and [$N_{\rm II}$] emission lines and not only to H$_\alpha$. Nevertheless through this paper we refer for simplicity to them as $f_{{\rm H}_\alpha}$ and $L_{{\rm H}_\alpha}$ respectively. The aim of the observations was to study the recent star formation rate occurring in HCG galaxies. Since the H$_\alpha+[N_{\rm II}$] emission is a good star formation tracer as well as the H$_\alpha$ line alone (Kennicutt & Ken 1983), the presence of [$N_{\rm II}$] does not invalidate our data. Nevertheless, since the H$_\alpha/[N_{\rm II}]$ ratio is not constant with radius in the largest galaxies, we will refer to the global star formation rate of galaxies, that is to the rate integrated over all the emitting area of each galaxy.

In Table 5 the journal of the observations is reported as follows:
Column 1: Name of the groups;
Column 2: Observing date (mm-yy);
Column 3: Central wavelength for the H$_\alpha^{\rm on}$ filter used (Å);
Column 4: Integration time for H$_\alpha^{\rm on}$ filter exposure (s);
Column 5: Central wavelength for the H$_\alpha^{\rm off}$ filter used (Å);
Column 6: Integration time for H$_\alpha^{\rm off}$ filter exposure (s).

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