2 Observations

The present observations were acquired using the 1.5 m f/13.5 Johnson telescope at the Observatorio Astronomico Nacional at San Pedro Martir, Baja California, Mexico. A Thomson $2048\times 2048$ CCD was used to obtain images of the source in [NII] $\lambda 6584$ Å, [OIII] $\lambda 5007$ Å, HI $\lambda 6563$ Å, and [SII] $\lambda\lambda 6717$ Å and 6731 Å at a plate scale of 0.4 arcsec pixel^-1.

Seeing was poor (of order 2 arcsec) but consistent, whilst flux calibration was performed using BD+$8^\circ$2015 and Kopff 27, whose continuum fluxes are quoted by Stone (1977). An alternative calibration was available in the case of the [SII] filters (used to derive the density map in Fig. 5), since the field stars have closely comparable fluxes at the wavelengths $\lambda\lambda 6731$ Å and 6717 Å of the [SII] doublet.

The images were subsequently processed using the Figaro package, and are presented in Fig. 1. In addition, the individual [SII] $\lambda 6717$ and $\lambda 6731$ images were ratiod with each other, and the resulting matrix converted into a map of projected densities illustrated in Fig. 5.

  

Figure 1: Direct imaging of NGC 2818 in the transitions [NII] $\lambda 6584$ Å, [OIII] $\lambda 5007$ Å, HI $\lambda 6563$ Å, and [SII] $\lambda\lambda 6717$ Å+ 6731 Å. Note the presence of E-W arm-like features extending on either side of the core, and the apparent increased levels of complexity in the lower excitation images