HUBBLE-SPACE-TELESCOPE WIDE-FIELD PLANETARY CAMERA-2 IMAGING OF THE CRAB-NEBULA .1. OBSERVATIONAL OVERVIEW

We present Hubble Space Telescope WFPC2 images of a portion of the Cra b Nebula supernova remnant obtained using narrow passband filters. The se images show a wide range of filament morphologies including small i ndividual isolated knots, long smooth arcs, and chaotic filament compl exes, with evidence that some filament cores are far denser and cooler than had been expected. Although several filaments show resolved ioni zation structures, in general such structure is not seen on a filament -by-filament basis. In areas where the filaments are mainly on the rem nant's near side, ''shadowing'' features are seen against the bright s ynchrotron emission background because of absorption by dust in the fi laments. While most of these dust features appear to be the cores of k notty-type filaments, we identify a remarkable sinuous dust shadow tha t maintains a coherent structure over similar to 25 '' (0.24 pc, assum ing d = 2 kpc). The obscuration caused by this narrow structure requir es surprisingly dense dust concentrations. The HST imaging data reveal relatively few emission-line structures that are unresolved at WFPC2' s 0.10 '' resolution, with characteristic angular knot sizes of 0.4 '' to 0.8 '', corresponding to physical scales of 1.2-2.4 x 10(16) cm at the assumed distance of 2 kpc. Such sizes are roughly in accord with expectations from theoretical models of the filaments. Future papers i n this series will concentrate on specific aspects of the data set in more detail.