7 Conclusions

New monochromatic images of 7 Abell PNe are presented. These images were absolutely calibrated, allowing the intensity distributions in different ions to be compared quantitatively. We concluded that 3 of the 7 Abell PNe are intrinsically spherical whereas the others have bipolar morphologies similar to the general PNe population. Haloes are detected outside of the shells of A 13 and A 24. We suggest that these haloes represent projections of the bipolar lobes on the plane of the sky. Jet-like, radial structures are also seen in A 24. The existence of a higher frequency of spherical morphologies among evolved PNe is still unexplained by current theories of PNe evolution.

Acknowledgements

CTH would like to thank the University of Calgary's staff for their kind hospitality greatly appreciated during his visit of the Department of Physics and Astronomy. Thanks are also expressed to Mme J. Martinis for her valuable assistance for using MIDAS/IDL facilities for image processing. The manuscript has been improved thanks to pertinent comments from Dr M. Rosado, the referee. This work is achieved for the GDR 968 "Milieux Circumstellaires''.