Panoramic views of the Cygnus Loop

We present a complete atlas of the Cygnus Loop supernova remnant in the light of [O III] (lambda 5007), H alpha, and [S II] (lambda lambda 6717, 6731). We include low-resolution (25 ") global maps and smaller fields at 6 " resolution from observations using the Prime Focus Corrector on the 0.8 m telescope at McDonald Observatory. Despite its shell-like appearance, the Cygnus Loop is not a current example of a Sedov-Taylor blast wave. Rather, the optical emission traces interactions of the supernova blast wave with clumps of gas. The surrounding intersteIlar medium forms the walls of a cavity through which the blast wave now propagates, including a nearly complete shell in which nonradiative filaments are detected. We identify nonradiative shocks around half the perimeter of the Cygnus Loop, and they trace a circle of radius R = 1 degrees.4 (19 pc) in the spherical cavity walls. The Cygnus Loop blast wave is not breaking out of a dense cloud but is instead running into confining walls. Modification of the shock velocity and gas temperature due to interaction of the blast wave with the surrounding medium introduces errors in estimates of the age of this supernova remnant. The optical emission of radiative shocks arises only where the blast wave encounters inhomogeneities in the ambient medium; it is not a consequence of gradual evolution to a global radiative phase. Distance measurements that rely on this uniform blast wave evolution are uncertain, but the radiative shocks can be used as distance indicators because of the spherical symmetry of the surrounding medium. The interstellar medium dominates not only the appearance of the Cygnus Loop but also the continued evolution of the blast wave. If this is a typical example of a supernova remnant, then global models of the interstellar medium must account for such significant blast wave deceleration.