An Atlas of Ancient Planetary Nebulae and Their Interaction with the Interstellar Medium

We present an imaging atlas of the largest planetary nebulae (>8'), taken at Hα, [N II], and [O III]. Using this data, we have developed a set of simple criteria for determining whether a planetary nebula (PN) is indeed interacting with the interstellar medium (ISM). On this basis, we conclude that most in our sample reveal significant interactions with the ISM. We discuss also how a large sample of ancient planetary nebulae can be used to derive a filling factor for coronal gas in the ISM.