3 The observations

The observations were carried out with the CCD imager at the Observatoire de Haute Provence 120-cm telescope at St Michel l'Observatoire. The image scale and field coverage offered by the f/6 aperture and the Tektronix $1024\times1024$ CCD provide 0.686$^{\prime \prime}$/24 $\mu$m-pixel and a large field of view of 11'7. Images were bias subtracted and flat-fielded in the usual way. The flat fields were generally obtained in twighlight to ensure that the illumination across the detector was the same as the sky illumination in the observations. We have also used a $3\times3$ pixel window filtering to remove detector artifacts such as dead or noisy pixels, after correction for cosmic rays.

  

Table 2: Derived properties

For A 30, the [N II] flux is for the inner area including the 4 knots. Values quoted in Cols. 5 & some of 6 are from Cahn et al. (1992) when available, otherwise we derived $c_{\rm H\beta}{\rm (radio)}$ from 5 GHz data and H$_\beta$ flux from Acker92. Angular dimensions are measured in arc seconds on the H$_\alpha$ images of observed PNe.

3.1 The interference filters

The interference filters used in the present study have very narrow bandwidths and are centred at specific wavelengths suitably selected (i.e. taking into account the f/6 aperture ratio of the 120-cm telescope and the possible temperature variation during the nighttime) for investigating nebular ionization structure and/or the PN abundance distribution mainly H$\alpha$ ($\Delta\lambda=8.7~{\mbox \AA}$), [N II]6583 Å ($\Delta\lambda=8.2~{\mbox \AA}$), and [O III]5007 Å ($\Delta\lambda=8.4~{\mbox \AA}$). Narrow band filters are necessary to determine specific contribution from individual ion. As a matter of fact, some PNe do show [N II]6583 Å emission much stronger than H$\alpha$'s ones (e.g. PN G 226.7+05.6 in Dopita & Hua 1997). These filters are mounted directly in front of the detectors, in the direct imaging mode.

3.2 The photometric calibration

The CCD responses were calibrated by observing the standard stars HD 93512 and HD 109995. The O-type star was used for calibrating the red fluxes (H$\alpha$ and [N II]6583 Å emission lines, to avoid the (weak) stellar absorption at the H$\alpha$ line), while the blue flux calibration used the other standard star. Such a calibration was satisfactorily controlled by using the absolute flux calibration provided directly by compact PNe's measurements (Dopita & Hua 1997; Hua et al. 1998). The image quality was typically $1\hbox{$.\!\!^{\prime\prime}$}5 - 2\hbox{$.\!\!^{\prime\prime}$}0$ during the observations. Nebular and standard star frames were processed and calibrated using either the IRAF or MIDAS software packages. The cleaned standard star frames then were used for the ADU counts/absolute flux conversion after airmass correction, sky subtraction and allowance for the filter bandwiths. Absolute fluxes (uncorrected for reddening) are given in erg cm^-2 s^-1 units (Table 2), with about 10% accuracy. However, further corrections for interstellar extinction might be eventually done using either optical or radio values quoted in Cols. 5 & 6 of the same table. In practice, once converted in absolute units, flux calculations were done in nebular frames with MIDAS INTEGRATE/APERTURE which allows to account for the actual contours of the planetary nebula (with 3$\sigma$) above the sky background. Such absolute flux measurements are useful since PN fluxes are proportional to the emission measures $N_{\rm e}^2V$ (of course, we still miss the "depth'' of the nebular volume, and the formula used assumes spheroidal structure!), it therefore provides quantitative density distributions of each of the observed ionic species.