We chose the Cygnus Loop because it offers a unique opportunity to learn the details of the interaction of a blastwave with interstellar clouds. We have been rewarded by an unprecedented view of the gas dynamic processes that occur when a blastwave strikes a cloud. The global view afforded by the ROSAT-HRI mosaic highlights the inhomogeneity of the ISM and the pivotal nature of blastwave-cloud interactions in determining the X-ray morphology of the Cygnus Loop. A combination of the ROSAT-HRI survey and a deep H mosaic provide definitive evidence that the Cygnus Loop was formed by an explosion within a preexisting cavity. A by-product of this study is an inventory of the interstellar medium in the vicinity of the Cygnus Loop.
Finally, this survey provides a unique asset for planning observations
which make full use of the very high spatial resolution of AXAF.